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How to Wire a 7 Way Trailer Plug (It's Easier Than You Think)

7-ways are some of the most common harnesses found on trailers. 7-ways provide the required running lights, turn signals, brake lights, and ground for the trailer. In addition, they provide three additional pins for a 12V hot lead, electric brakes, and reverse lights.Trailer wiring can be one of the most intimidating components of your towing setup, but it doesn't have to be. Most of us aren't electricians, but that doesn't mean wiring a trailer is beyond us. We'll walk you through the trailer-wiring process for 7-way plugs below, so you can get your trailer up and running in no time.
Watch vehicle-side 7-way installation
Watch trailer-side 7-way installation

What is the Color Code for 7-Way Trailer Wiring?

Before we get into the step-by-step walkthrough, we'll first go over the standard color code for 7-way wiring harnesses. This will make it easier when it comes time to make your wiring connections.The color code for wiring harnesses varies between industries. In particular, SAE (Society of Automotive Engineers) standard wiring differs from the RV industry's standard wiring. The standard for both is as follows:SAE Standard
  • Green: Right turn/brake light
  • Yellow: Left turn/brake light
  • Brown: Tail/running lights
  • White: Ground wire
  • Blue: Brake controller output
  • Black: Battery hot lead
  • Purple: Reverse lights
RV Standard
  • Green: Tail/running lights
  • Yellow: Reverse Lights
  • Brown: Right turn/brake light
  • White: Ground wire
  • Blue: Brake controller output
  • Black: Battery hot lead
  • Red: Left turn/brake light
Not sure exactly what each wire does? The easiest way to figure it out is to use a circuit tester to confirm the function of each wire. What Size Wire Gauge is Used for a 7-Way Wiring Harness? The minimum suggested wire size for a 7-way trailer plug is 16 gauge for the turn signals, brake lights, reverse lights, and running light wires. The suggested minimum for the ground, brake power, and battery hot lead wires is 12 gauge.
7-Way Flat SAE Connector Diagram
7-Way Plug RV Standard Diagram
7-Way Plug Heavy Duty Diagram
7-Way Trailer Wiring Color Code Traditional SAE
7-Way Trailer Harness Diagram - Traditional SAE
7-Way Trailer Wiring Color Code RV Standard
7-Way Trailer Harness Diagram - RV Standard

Wiring a Trailer with a 7-Way: Step by Step

Now that we have an idea what our wires do, let's get into the step-by-step process of wiring your trailer with a 7-way connector.

Trailer Side

Step 1: Prepare for Trailer Wiring Installation

Start by making sure you have everything you need to wire your trailer, such as:
If you need to replace one or more of your lights, you can purchase trailer lights or use a complete trailer light kit, which will come with the necessary wiring included. Remove old lights before beginning the new installation.If you don't need to replace your lights, you can simply use a 7-way harness.
Trailer Junction Box
Pictured: junction box

Step 2: Locate or Install Junction Box

If you're replacing existing wiring and your trailer already has a junction box, locate it (it's typically near the front) and remove the cover. Remove the nuts from each terminal.If you're installing a new junction box, find a suitable location for installation, such as on the inside edge of your trailer tongue. The box should be close enough for your wires to reach, but away from any components it could interfere with.If you don't have a junction box and don't want to install one, you can simply tie the new wiring harness in with your trailer's existing wiring.
Junction Box on Trailer
Locate the junction box and remove the nuts from each terminal.

Step 3: Make Trailer Connections

NOTE: Check the manufacturer's instructions for any wiring harness you use. The instructions will identify the function of each wire. You won't always be able to rely on color alone to match up wire functionality.
WITH JUNCTION BOXIf you have a junction box and are simply adding a new 7-way connector, remove the nuts on each junction box stud. If you're replacing existing wiring, go ahead and remove the old wiring now by removing each terminal from the junction box.Cut any excess wiring from your new cable and strip the wires of insulation using a crimper. Next, crimp ring terminals onto the new wiring. Place the terminals onto their corresponding studs in the junction box (ground wire to ground wire, brake wire to brake wire, etc). When finished, reinstall the nuts.Secure any excess wire with wire clips.
Trailer Junction Box - Remove Wiring
Trailer Junction Box - Strip Wire Insulation
Trailer Junction Box
Trailer Junction Box Wired
WITHOUT JUNCTION BOXIf you're not using a junction box and are simply connecting the new 7-way to your existing wiring, you can use butt connectors and a heat gun to make your wiring connections.Secure any excess wire with wire clips.
Butt Connector Trailer Wires

Vehicle Side

Now that our trailer is hooked up, it's time to wire our vehicle. Let's get started:
Step 1: Prepare for Vehicle Wiring InstallationIF YOU HAVE A 7-WAY CONNECTORIf your vehicle already has a 7-way connector, then great! Simply plug the trailer-end connector into the vehicle-end connector, and you're ready to roll.IF YOU HAVE A 4-WAY CONNECTORIf your vehicle has a 4-way connector, the easiest way to add a 7-way is with an adapter kit like the ETBC7, which includes everything you need to convert your 4-way into a 7-way. Confirm the function of your vehicle wires with a circuit tester prior to connecting. Activate each function (turn signal, brake lights, etc.) in turn and check for a corresponding signal from the circuit tester to make sure you have the right wire.Follow our detailed instructions on installing an ETBC7 kit here.IF YOU HAVE NO CONNECTORIf your vehicle lacks any kind of connector, the easiest way to install a 7-way plug is to install a 4-way and use an adapter. For help installing a 4-way connector, view our how-to guide here. For detailed instructions on installing an ETBC7 kit, check out our guide here.Also take the time to confirm the function of your vehicle wires if you are splicing or clamping wires together. Check your vehicle's owner manual or use a circuit tester to confirm each wire's function prior to connecting. You can usually tap into your vehicle's wiring system with a circuit tester at the rear of the vehicle, behind the taillights. Activate each function (turn signal, brake lights, etc.) in turn and check for a corresponding signal from the circuit tester to make sure you have the right wire.
4-way to 7-way adapter
If you have a 4-way plug, add a 7-way with a 4-to-7-way adapter
Circuit Tester in Use on Trailer
Use a circuit tester to confirm wire function
Step 2: Make Vehicle ConnectionsYour wiring harness will either plug into, clamp onto, or splice into your vehicle's existing lighting. The type of connection depends on what is available for your vehicle model. We'll go over each process in more detail below.Some general notes and tips:
  • If the vehicle connector is under the vehicle, use a mounting bracket to attach it to the vehicle. This will help prevent damage that may occur if the connector is left dangling.
  • Use a small amount of grease on all electrical connections—the plugs on your automobile and the connector itself—to help prevent corrosion.
4-WAY TO 7-WAY ADAPTERSAn adapter will plug into your existing 4-way plug and provide two additional pins for your trailer brakes and battery lead. We've put together adapter kit # ETBC7, which includes everything you need to install a 7-way adapter, including the connectors, wiring, and circuit breakers.Once you plug the adapter into your existing 4-way, you will have an additional three wires that must be connected.One of these wires (typically blue but sometimes black) will run to your electric brake control power output. Another wire (typically black or red) will run to the positive terminal of the battery via additional wire and butt connectors.Use a circuit tester to test for wire function before connecting.
How to Wire Electric Trailer Brakes Diagram SAE
How to Wire Electric Trailer Brakes Diagram RV Standard
Watch video installation of 4-way to 7-way adapter installation
PLUG-IN STYLESome vehicle manufacturers essentially "pre-wire" your vehicle so that your wires are easily accessible for connection. T-connectors such as this one simply plug into your vehicle's existing wiring, no cutting or splicing required.The plug-in location is typically near the taillights, underneath the vehicle, or behind the paneling in the back cargo area. You may have to remove your trim access cover, scuff panels, taillights, etc. in order to access the plug.
Watch video installation of T-connector
SPLICE-IN STYLEHardwire kits aren't quite as convenient as the other styles, but splicing into your vehicle wiring is actually less difficult (and scary) than it sounds.After confirming your wire functions (using your owner's manual or circuit tester), connect your wires using one of three methods.Soldering: This is the best way to connect wires. Simpy solder your wires together for the strongest, most reliable connection using a soldering gun. Use heat shrink tubing to protect the soldered connection.Butt Connectors: If you aren't comfortable soldering wires, heat shrink butt connectors and a heat gun are the next best thing.Quick Splices: The quickest, easiest way to connect wires is by using a quick splice. Quick splices force a metal piece into two separate wires, thus connecting the circuit. NOTE: Quick splices are the easiest—but least reliable—wire connection method.
Trailer T-Connector
When FinishedAfter wiring up your trailer and vehicle, it's a good idea to plug the two ends together and test the function of each wire. Make adjustments as needed. If everything lights up when it's supposed to, you're good to go!
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Hi, I just purchased a Tekonsha Prodigy P3 Trailer Brake Controller with OEM Harness for my 2001 GMC Yukon that has the factory trailering package and 7-way connector. I've used it with other trailers with only 4-pin connectors (via an adapter), and all necessary functions work. I've installed the Brake controller in my vehicle and connected it to the fuse block. It powers up OK. I have yet to test it with my trailer brakes. But as I was also looking to install the Furrion Vision S Wireless RV backup Camera System I also bought from etrailer, I was testing out the connections on the 7-way and can't get any reading on the 12-volt power (hot) connector on the 7-way on the Yukon. I've tried it with it running, with Acc On, with lights on, and nothing seems to energize it. I can't find a fuse in the front left (under-hood) box for it, or in the inside panel on left side of dash. I want to use this circuit to both charge the trailer battery when towing (which I thought it would by default) as well as, possibly power an additional backup camera I want to install for the truck itself (not the Furrion, which is prewired on the trailer, and which I assume at this point, will work as expected). Am I missing something? Where should I look for a fuse? And is this circuit HOT all the time, or only when the ignition is ON or the engine is actually running? Any advice would be appreciated. Thanks!

Joe H.


will the hot lead charge an auxiliary battery or is a charger required

Etrailer Expert

Jenny N.


@JoeH You can maintain the charge of an auxiliary battery when driving by running a # 10-1-1 wire along with 40 amp circuit breaker # 9510 in-line to the black wire (12 volt hot lead); to keep that maintenance charge going only when your tow vehicle is running you will need to use solenoid # PK5231201. If you want to fully charge the auxiliary battery on your trailer the best option is to use a battery charger such as the CTEK MULTI US 3300 12-Volt Universal Battery Charger # CTEK56353 which you would plug into a standard outlet.

Joe H.


@JennyN thanks, the harness isnt protected.... 40 amp seems large for #12 ?
Etrailer Expert

Jenny N.


@JoeH The circuit breaker is installed close to the vehicle battery it is rated for the higher 40 amperage. It is true that by the time it reaches the back of the vehicle it would be lower, but if you placed a lower circuit breaker up by the battery it would trip at times when you really do not want that happening.
See All (7) Replies to Joe H. ∨

Scott E.


I am putting a battery on my horse trailer to run lights with or without a tow vehicle. I also have a trailer breakaway system already installed that needs to be incorporated (would I keep the existing battery for the breakaway or can the deep cycle battery be used?). Is there a wiring diagram available for this setup? Assume a diode would go somewhere to prevent power from feeding back to tow vehicle.

Etrailer Expert

Katrina D.


@ScottE Yes, you would definitely want to use your existing battery for the breakaway. I have spoken with one of our installers and he recommended getting two diodes for your set up; one for the battery itself and one for the breakaway kit. I have an image attached for your reference. I also have a link below to some instructions on the Progressive Dynamics Converter, part # PD4135K. This should include additional information on how to incorporate your existing breakaway system, so you have the ability to run lights without a tow vehicle included.

Billy V.


I have a utility trailer with a 4 way and I use a 7 way adapter to plug in my car. I built an enclosed box on the trailer and I want to be able to run a 120 outlet inside for a small freezer and a couple of lights. How do I go about doing this?

Scott A.


I just purchased a used travel trailer. 7 pin connector on trailer and my F150. When connected only the tail/running lights and trailer brakes work. No brake or turn signal lights on the trailer and it blows the mini cartridge fuse on my truck for those functions. I know it has been rewired with the trailer 7 pin being replace. I have checked for loose wires/shorts and haven't found any. The yellow wire from my truck is supposed to be for left turn signal/brake lights. The yellow wire from my 7 way trailer is not connected to anything. But the yellow wire on rv standards is back up lights. I didn't know there are 2 standards for 7way connections. Is it possible for my truck (SAE standard plug) wired to my trailer (RV standard) be the problem? Did someone wire it using the wrong standard? I'm confused, any suggestions?

Etrailer Expert

Mike L.


@ScottA There are two different wiring conventions for a 7-way. You can check them out using the provided link. The best way to confirm how your trailer is wired is to get a 12V battery, ground the negative post to the trailer frame and apply 12V to each socket of the 7-way to see what functions activate. At that point, you'll know whether everything's wired so it's compatible with the vehicle or not.



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