Trailer Wiring Diagrams

Trailer Wiring Connectors

Various connectors are available from four to seven pins that allow for the transfer of power for the lighting as well as auxiliary functions such as an electric trailer brake controller, backup lights, or a 12V power supply for a winch or interior trailer lights. Choose a connector that has the required number of pins for the functions required for your trailer. If the connector is under the vehicle, you will want to use a mounting bracket to attach it to the vehicle. This will help prevent damage that may occur if the connector is left dangling.

This chart is a typical guide, wire colors may vary based on manufacturers. Use a circuit tester to verify connections.

*NOTE: The fifth connection is sometimes used on 5-ways to power a reverse lockout on trailers with surge brakes. When this is the case, the lockout should be connected to the backup light circuit of the tow vehicle.

4-Way Connectors

4-Way connectors are available allowing the basic hookup of the three lighting functions (running, turn, and brake lights) plus one pin is provided for a ground wire. Most standard light duty trailers will use a 4-pole flat connector.

4-Way Flat Trailer Connector

4-Way Flat Trailer Connector

4-Way Round Trailer Connector

4-Way Round Vehicle Connector

5-Way Connectors

5-Way connectors are available allowing the basic hookup of the three lighting functions (running, turn, and brake) and, besides the ground, one pin is available to provide support for another function. Typically the 5-Way Flat is used for trailers with surge brakes or hydraulic brakes. The additional wire is tapped into the backup lights to disengage the hydraulic trailer coupler (actuator) when the vehicle is reversing, thus turning off the trailer's brakes.

5-Way Flat Trailer Connector

5-Way Flat Vehicle Connector

5-Way Round Trailer Connector

5-Way Round Vehicle Connector

6-Way Connectors

6-Way connectors are available allowing the basic hookup of the three lighting functions (running, turn, and brake). The ground and two extra pins are available to provide two additional functions, typically for electric brakes and 12 volt "hot" lead. The 6-way round connectors are very common on horse trailers. The 6-way square connectors are more common on campers.

6-Way Trailer End

6-Way Vehicle End

6-Way Square Trailer and Vehicle Ends

Note: In some applications, the black battery power connection and the blue brake controller output connection are reversed.

7-Way Connectors

Aside from the three main lighting functions, additional pins for electric brakes, a 12 volt "hot" lead, and backup lights are available. There are two types of 7-way connectors. One has flat pins, which are often referred to as blades. The other has round pins. The round pin style is very rare. The RV style 7-way with flat pins (or blades) is very common. It is often found on newer trucks and SUVs that come equipped from the factory with a trailer hitch.

7-Way Round Trailer Connector (Flat Pin)

7-Way Round Vehicle Connector (Flat Pin)

7-Way Round Vehicle Connector (Round Pin)

Note: To ensure that your trailer connector is wired properly, we recommend using a circuit tester to match pins by function. Because wire colors can vary, they are not as reliable for determining functions. And improperly connected wiring can cause damage to electric components.

Mounting Your Trailer Wiring Harness

Often the 4-pole trailer connector will remain in the trunk or cargo area of a car or SUV when not in use. This helps to extend the life of the connector by protecting it from the elements and accidental damage. When it is needed for towing, simply pull the connector out and shut the trunk or rear door. The rubber weather strip that provides a door seal keeps the wire from getting pinched. If the trailer connector needs to be mounted under the vehicle, we offer many different mounting brackets that will help to protect the connector and keep it from dangling beneath the vehicle.

Trailer Wiring Adapters

The T-One connectors and hardwire kits all use a 4-pole trailer connector. This connector is most common among the smaller utility trailers and can easily be adapted to match the larger 5-pole, 6-pole and 7-pole styles. Using an adapter lets you avoid having to splice into the vehicle's wiring system. Adapters will plug into the flat 4-pole connector and have wire leads to provide additional functions such as powering trailer brakes, power lead for utility lights, reverse, or auxiliary power for a winch or tools. If your vehicle or trailer is equipped with something other than a 4-way plug, such as the larger 7-way round plug, you can use our adapter fitguide to find the one that is right for your vehicle and trailer.

Troubleshoot Your Trailer Wiring

Watch now: Troubleshoot modulite wiring

If you have a Modulite (powered converter), Converter, or T-One connector that is not working properly, you need to use a circuit tester to determine the source of the problem.

Check to make sure there is a signal going into and coming out of the converter or Modulite box without the trailer hooked up. The only part of the wiring harness that will typically go bad is the box, so you need to determine if a signal is making it to the box, and if so, is it coming back out on the correct wires. There are four possibilities:

Problem: There is no signal going into the box, meaning something is incorrect on the vehicle.Solution: Solve the problem by checking for blown fuses and then check the installation points. Use a circuit tester to check that the correct wires are tapped into, the wiring harness connectors are plugged into the correct vehicle connectors, and check that connectors are plugged in all the way.
Problem: If a signal is going in and coming out on the appropriate wires, then there is a problem with the trailer wiring. Solution: Solve the problem by inspecting the wiring on the trailer to make sure all of the connections are correct and ground wires are connected properly. Most likely, the ground wire on the trailer is not secured properly. A trailer wiring system is grounded to the frame near the coupler and each light also needs to be grounded. If there is not a white ground wire coming out of a trailer light, then the light is grounded through its mounting studs. Too much paint, dirt, or rust can cause bad grounds, so make sure the ground is secured to the bare metal frame. Also look for pinched or cut wires; these are often found when wires are routed above suspension components or behind taillights.
Problem: If a signal is going into the box and not coming out or a signal is going into the box and coming out on the wrong wires the ground on the vehicle may not be connected properly. If the white ground wire is not installed properly on the vehicle, then the green and yellow wires will not carry signal coming out of the box. Solution: Solve the problem by checking to make sure the ground wire is installed properly on the vehicle. To make sure you have a suitable ground, first connect the circuit tester's ground wire to the same location as the ground wire on the wiring harness. Then test the signal going into the converter or modulite box. If there is no signal, connect the circuit tester to another grounding location such as the exhaust pipe. If a signal is present, the grounding location for the wiring harness is bad, and a new location needs to be found. On a few vehicles, the frame and body components are not grounded. In these rare instances, look for a ground wire coming out of the license plate light.

*Due to extensive testing and quality control, it is very rare that a new wiring harness will be defective. If the wiring harness worked properly when installed but then went bad, there is a strong possibility that it has shorted out because of a problem with the trailer wiring. Make sure to correct any trailer wiring problems before installing a new wiring harness.

Issue: Converter Shorting Out

When too many amps are drawn through the converter box, it can be shorted out. The possible causes are:

Too many lights on the trailer. Each incandescent taillight draws about 2 amps and side marker lights typically draw about 0.5 amps each. Most converters allow up to 4 amps to pass through them. A standard converter cannot have any more than 1 taillight on each side.
Water. Trailer connectors get wet, causing too many amps to be pulled through the converter. Water can unite all 4 wires on the trailer connector causing it to draw too many amps. This often happens when a boat trailer is backed into water with the trailer wiring still connected to the vehicle. To solve the problem of connections getting wet, always disconnect the trailer connections before backing into the water. Also, covers can help prevent water from getting into trailer connectors when it rains or when a vehicle is taken through the car wash.
Bare wires touch each other or the trailer frame. This causes too many amps to be pulled through the converter box. The coating on trailer wires can wear thin, often at the connection to the trailer's taillight or where the wires pass over the trailer suspension. If wires are hung too loosely, they can get pinched and smashed between the trailer frame and suspension U-bolts. To solve the problem, inspect the trailer wires and replace any that are damaged or frayed.

Issue: Weak Ground

Watch now: How to Wire the Ground

If you find some, but not all, lighting functions work properly you may have a ground problem. In this situation a weak ground is just strong enough to provide some lighting functions but not all. When the taillights and brake lights are used at the same time, it creates the maximum amp load of the lights on the trailer. If a ground problem exists, it will show at this point. The places to check grounds are as follows:

The connector on the vehicle should have a ground wire secured tightly to a clean surface on the frame. A wire attached to the body or a surface with undercoating or rust can cause a ground problem.
The connector on the trailer should have a wire from the connector secured tightly to a clean surface on the trailer.
Each tail lamp assembly at the back of the trailer also must be grounded. This is done in one of two ways. The first is by a separate wire coming from the back of each tail lamp assembly being secured to the frame. The second, and most popular, ground is achieved through the bolts of the tail lamp assembly. In this case, the lamp housing must be attached to the frame of the trailer. If the lights are mounted on wood or PVC material, the light will not receive ground.
The last possibility to explore is in the trailer design. If the trailer has a tilting bed, it is possible the ground is not passing through the pivot point, resulting in a poor ground. The solution to this is to route a ground wire from the connector at the front of the trailer to each lamp assembly at the back of the trailer, bypassing this pivot point.

NOTE: If you purchased your wiring harness from etrailer.com and none of these fix your problem please feel free to contact us so our product experts, and installers can determine possible remedies or warranty coverage.

How to Wire your Car or Truck to Pull a Trailer

By law trailers are required to have at least running lights, turn signals, and brake lights. To provide the power and a connection for these functions, the tow vehicle's electrical system needs to be tapped into. There are two options available for connecting to your vehicle's electrical system, a plug-in style T-One connector or a custom hardwire kit.

T-One Connector

The easiest way to make this connection is with the use of a T-One Connector, which comes with OEM style connections that simply plug into your vehicle's existing wiring harness, typically near the rear of the vehicle or by the tail lights. Sometimes the vehicle manufacturer will run the wires to an easily accessible plug underneath the vehicle or behind the paneling in the back cargo area. T-Ones come pre-wired with a 4-way flat trailer connection and can be expanded to 5-way, 6-way, or 7-way trailer connections through the use of a wiring adapter.

Custom Hardwire Kit

If a T-One is not available, a connection can still be easily made by using one of our custom selected hardwiring kits. We offer kits with all the pieces you will need to simply tap into the existing wires on your vehicle. This may sound difficult, but quick splices make installing the wiring harness quick and easy. A quick splice has two grooves in it; one groove is for the vehicle wire, and the other groove is for the wire on hardwire kit. Once both wires are in the grooves, you simply press down on the top of the quick splice. This forces a metal piece into both wires, connecting the circuit and eliminating any need for cutting or splicing.

To hardwire the tow vehicle for a trailer connector you need to locate the proper wires. To help in this task you can check the vehicle's owner manual or use a circuit tester. The circuit tester is used to make sure the correct wires on the hardwire kit are connected to the matching wires on the vehicle; it helps determine which wire performs which function. The easiest place to tap into the vehicle's wiring system is behind the tail lights. By turning on the left turn signal, a circuit tester can be used to test the different wires behind the driver's side tail light. When the circuit tester lights up, you know that wire carries the left turn function. A quick splice slid through the yellow wire on the trailer connector and around the just found vehicle wire will clamp down to provide a secure connection.

The three types of hardwire kits are:

Standard 4-Pole Wiring Harness - For use with vehicles that have adequate power and standard wiring system, these simply connect into existing wires on the vehicle and have a 4-pole flat connector to attach a trailer.
Converter - For use with vehicles that have separate turn and brake light wires. Some vehicles send only one signal per wire, creating what is called a 3-wire system: one wire for the left turn, one wire for the right turn, and one wire for the brake signal (common on vehicles with amber turn signals). A converter will reduce it to a standard 2-wire system needed for wiring a trailer. There are still additional wires for the running lights and for the ground. Any vehicle with amber turn signals will need a converter. However, there are some vehicles with all-red tail lights that can also require a converter. A wiring harness with a converter has a black box built in it. Five wires go into the box, and only 4 come out. The converter transfers the brake signal on the vehicle into the left and right turn signals for the trailer wiring system.
Modulite or Powered Converter - Used with vehicles that do not provide enough electrical power to handle the additional strain of powering trailer lights, the Modulite or powered system draws power directly from the battery but still connects to the vehicle's wiring system to determine when to power the lights on the trailer. If there are too many lights on the trailer for the vehicle's electrical system to provide adequate power, use a modulite or powered converter. A modulite installs the same way as a standard converter except an extra wire must be run to the battery. Instead of drawing power from the vehicle wiring system, a modulite draws power directly from the vehicle battery. This is safer because the extra amps to power the trailer are no longer going through the expensive electrical components of the vehicle. More vehicles are using thinner gauge wire and require a modulite, regardless of how many lights are on the trailer, simply to protect their wiring system.

Vehicle manufacturers have intermittently changed wiring colors over the years. We recommend you use a circuit tester on the tow vehicle's wiring harness to verify that the correct wire has been located for the proper function.

Questions and Comments about this Article

Shannon 5/5/2021

How can I run a 12v fuel pump into a 7 pin and control the power from a uplifter switch in the cab of my truck Ford F-250 super duty

Reply from Jon G. 5/6/2021

I'm not sure how you would make the connection to your uplifter switch and you would want to make sure that you have a circuit breaker or fuse to protect it, but what I would do is run 10 gauge wire # 10-1-1 from your uplifter switch back to the 12V power pin on your 7-Way which would be the pin in the 1 o'clock position .

Reply from Shannon 5/6/2021

@JonG ok thanks I’ll try it

Steve 5/3/2021

I have a 2020 G M C truck I wonder can I use the light wire to charge my battery to my power winch it a lawn mower battery ? If I turn my lights on while running ? That the flat 4 plug .

Reply from Jon G. 5/4/2021

The 12V power wire struggles enough to charge a battery with a 10 gauge wire so the smaller 16-18 gauge wire used for your trailer lights will not be nearly large enough to give your trailer battery a trickle charge. Your truck should have a 7-Way socket as well so all you should need to do is install a 7-Way on your trailer and route the power wire to your trailer battery. I recommend the kit # e99011 because it will help keep your wiring organized and make future repairs or modifications much easier.

Reply from Steve 5/4/2021

@JonG thank you

Reply from Jon G. 5/5/2021

@Steve You're welcome!

Ben 4/30/2021

Without turning the mini van on I have full lights on, brake, left and right work perfectly. New cable harness. I need to disconnect the trailer connector to kill the running lights. Any ideas

Reply from Jon G. 5/3/2021

It sounds like you have LED lights on your trailer. LED Lights are nice because they don't require as much power to operate but as a result sometimes they will light up dim when there is the smallest amount of power on those circuits. If you use a resistor like part # C57003 for a 7-Way setup or some that you install on the wiring directly, like part # DI34ZR , it should take care of your problem.

Bob C. 4/20/2021

Hi. What is the best way to add a third brake light on a camper shell mounted on a 2002 GMC Sonoma SLS? I have a junkyard rescue shell that does not have a third brake light and will mount an aftermarket LED but I am not sure if a hot wire is available. Just need advice from more knowledgeable folks

Reply from Jon G. 4/22/2021

Sometimes there will be provisions for 3rd brake light wiring somewhere along the frame but from the research I've done in the past it seems like there is no rhyme or reason as to which trucks have this wiring and which trucks don't. The best solution I have for you is to install the 3rd Brake Light Logic Module # PP20-702 . This is a very straightforward installation and will give you that brake signal that you're needing.

Edward R. 4/18/2021

Hi! I have a 2012 RAM 1500 quad cab which has the standard incandescent lights. I’m building an Wind Stream 24ft utility trailer with LED LIGHTS. Question is “Do I need to have a converter to control the return voltage to control and protect the fuse panel in the truck?”

Reply from Jon G. 4/20/2021

If you don't have the factory 7-Way and you're just wanting to install a 4-Way flat trailer connector for the trailer lights then I wouldn't say you HAVE to have a converter, but it does help to protect your truck so I do recommend using a wiring kit that comes with one like the part # 119178KIT .

Craig W. 4/16/2021

I need to wire a four flat to the back of my camp trailer in order to connect to small aluminum trailer which already has a four flat. Any advice on this project would be helpful.

Reply from Jon G. 4/20/2021

You can try to tie that into your current trailer wiring with a 4-Way flat like part # 18252 . One thing to be mindful of is that this will pull more power on those trailer light circuits so you might see problems there if there is too much power being pulled.

Matt S. 4/2/2021

Is there a ASME, NEMA or other standard that controls the pin diameter and spacing of 4-flat plugs? I have several trailers with 4-flat plugs and the spacing and diameters, while similar, do not appear to be the same, which makes it difficult to plug into some and loose connections on others.

Reply from Jon G. 4/3/2021

I don't think there is a specific standard when it comes to the spacing and sizing for the 4-Way flat plugs - I think it's more of a "make your plug to fit like this so that it works with everyone else" sort of thing. You definitely aren't alone with this situation, which is why Hopkins came up with their Easy-Pull Harness that has a ring in the middle of the connector that makes it easier to remove when you need to. For a trailer end piece use part # HM48114 and for the vehicle end use part # HM48044 .

Butch 3/29/2021

I have a 2001 Ford f250 super duty truck, I just purchase a sure trac enclosed trailer and moved to Florida. All the lights work on the trailer however I cannot get the Interior dome lights to work. I watched a you tube video to where a man said that his Chevy truck did not have the fuses installed in the fuse box, can anyone tell me what fuses I need to check and have installed? Any help would be appreciated.

Reply from Jon G. 4/2/2021

You should be able to find this location in your copy of the owner's manual. When I looked online I believe the fuse you're looking for is in location 16 and is labeled as "Trailer Tow Battery Charge" and it sends back 12V power. You'll want to confirm this on your end too.

Reply from Steve 4/12/2021

@Butch My trailer dome lights only work when the truck running lights are on.

Willie J. 3/28/2021

Hey, I just brought a used Carry-On trailer with a 7way flat and it have three wires on the left side and two wires on right side running from the rear for clearance marker and tail lights . With only one oval LED fixture at rear on each side. Do I need a 5 to 4 converter? My truck is 2010 Ford F-150 with turn/brake is feed from the same wire.

Reply from Kef G. 4/1/2021

It sounds like your trailer has a 7-way blade, and your F-150 has a 4-way flat. What you need to do is upgrade your 4-way flat to a 7-way blade with part # ETBC7 . This will give you the 7-blade trailer connector you need.

Bill F. 3/25/2021

Need to go from 4 wire flat on motorcycle to 9 wire round on a can am motorcycle trailer. It is a 2018 Honda motorcycle.

Reply from Kef G. 3/25/2021

There aren't any 4-way to 9-pole adapters, so the best way to get this working is to splice a 4-way into your trailer and remove the 9-pole.

Drew M. 3/16/2021

I have 6 wires going into a 7 wire plug. None of the wires are blue and my electric brakes do not work. The only blue wire I see goes to a junction box on trailer w/black wire. How should I wire the plug? Should the black act as the blue wire? Very frustrating

Reply from Jon G. 3/18/2021

While this article shows industry standards, it isn't a hard and fast rule. It sounds like you'll need to test the functions of all of your wiring and make your connections per function instead of per color.

Richard H. 3/14/2021

I am rebuilding an old aluminum trailer and have purchased new replacement sidemarker lights. Because the metals react to each other and corrode, I have put a silicone gasket between the two metals. Will this cause a grounding issue? I have done the same with the new taillights. Thx!

Reply from Jon G. 3/15/2021

If the grounds for the lights are not able to make contact with the trailer frame then yes, that will cause issues. To try to avoid the corrosion you're talking about I would try to ground all of the lights to a main ground line and route that up to your trailer connector.

Dan O. 3/6/2021

Question, not sure if this is the correct location to ask a question. But here it is. I have a 1982 Sunline camper trailer. I replaced the standard bulbs with new LED tail and running lights. Their was green and white wires at running light locations. Here’s the issue, when the running lights are OFF the turn and stop lights work fine. When the running lights are ON the turn and STOP lights DO NOT work.

Reply from Jon G. 3/8/2021

Sounds like you either have a bad ground or short in your wiring. Check and make sure that all of your wiring (on both sides of the towing setup) is in good condition and that all of your grounds are secured to a clean, bare-metal surface. Attached is a link to a tiring troubleshooting article that you can check out.

Reply from Roger K. 4/10/2021

@DanO I also ran into a simular problem with my vehicle when I converted to LED lights. I found the problem to be that the vehicle had a negative switching system which does funky stuff to the lights. I wired in relays to convert the negative switching to positive switching now I am having no issues.

Emmanuel A. 3/2/2021

Hi there a have a small car dolly 7pins plug and the colors the my has is yellow Lt, white GD, green Rt but i am confused theres is one wire yellow/brown and green/brown i dont know what terminal are those TM or EB or AX. I need some help please. Thank you

Reply from Jon G. 3/4/2021

What you're going to need to do is apply 12V power to those wires and see what happens when you do that. Brown is typically used for running/tail lights so that might be where those wires need to go, but you also need to make sure that you have the stop light covered as well (which is typically on the same circuit as the turn signal). The one annoying thing about wiring is that there are typical colors but it isn't an industry standard that everyone has to follow. This means if something doesn't quite match up (like what you're experiencing) then we need to do the testing.

Dave H. 1/28/2021

I really don't like the Idea of "showing" electric trailer brake wire being grounded to the trailer axle. My axles move and is some what going to have a voltage drop when finding ground. My preferred method is to connect to the ground wire and keep the frame ground out of my schematics. By adding undetermined amount of resistance to the brakes you may end up with each wheel or at least each axle getting a different current flow and uneven braking. Thank you for having all this information quick and easy to find.

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