Routing the Power Wire for a Trailer Wiring Harness


Some trailer wiring harness 4-Way and 7-Way connectors require a power wire to be routed from the harness to the vehicle battery. The power wire is required on trailer wiring harnesses for some vehicles because the electrical system on those vehicles cannot handle the amperage draw associated with trailer lights. The power converters and ModuLites used in these applications connect to the vehicle battery to provide the power for the trailer lights, protecting the vehicle's sensitive electrical system. On vehicles that require a 7-Way connector, a power wire will typically be routed to the battery via a 40-amp circuit breaker to allow the 12-volt circuit in the connector to function. Note that the power wire should not be connected to any power source other than the battery. For instance, you should never connect the power wire to the 12-volt socket found in some vehicles' cargo areas. Connecting the power wire to an alternative power source could overload the circuit and damage your vehicle's wiring.


Modulite wiring kit Labeled wiring connector




How Do I Route the Wire?


First, you need to know where the battery is located. Typical locations include:


Once you know where your battery is located, you can decide which of 3 methods will work best for you. There are 3 ways or paths you can take when routing the power wire to the vehicle battery.


Each method has its pros and cons. You can use any method, or a combination of methods, for the results that suit your specific needs and application.





Method 1: Under the Vehicle


The most basic method is also the fastest and easiest way to route the power wire from your wiring harness or 7-Way to the battery. First you should follow the installation instructions for the wiring harness or 7-Way leading up to routing the power wire.


In most vehicles, the power converter will be located in the trunk or cargo area or behind a tail light of the vehicle. On some vehicles, the power converter is mounted underneath the vehicle on the outside. A 7-Way is mounted outside of the vehicle at the back.


Read in more detail about how to install a 7-Way


Grommet and opening in car body - for wire routing Underside of auto body - for routing wiring




Wiring ties fuse holder Battery post of circuit breaker



The benefits of this method are that it is fast and easy. A drawback is that the wire is not as protected from the elements as it could be.





Method 2: Through the Frame


The second method will help protect the wire better than leaving it exposed under the vehicle. This method requires that you route the power wire through the frame of the vehicle toward the front where the battery is located.


Routing the wire through the frame can be tricky. Every frame is different, so this method may or may not be an option.



Underside of car body


The benefit of this method is that the wire will be more protected from the elements. The drawbacks are that installation is not as easy, takes longer, and makes it more difficult to troubleshoot the wire should something malfunction. Also, some frames are only partially open, so you would need to weave the wire in and out of them.





Method 3: Through the Vehicle


The third method of routing the power wire involves running the wire through the vehicle interior. This is usually a must if the battery is located in the trunk or cargo area or under the backseat. Every vehicle interior is different, so the path you choose to take is up to you.


Usually, this method requires that you route the power wire first through the trunk or cargo area. For a 7-Way installation, unless the battery is located in the interior of the vehicle, Method 1 or 2 is recommended.


You can then loosen or remove some interior trim panels or carpeting to hide the wire before entering through the trunk to the backseat area of the vehicle.



Wire routing path - trunk through back seat

From this point there are several options:


Once you have reached the front of the vehicle, you can route the wire through the firewall and into the engine compartment. When you go through the firewall, you may find an existing wire bundle going through a grommet that you can use.


You may find that you will need to drill through the firewall. If you do have to drill, use caution and do not drill through any components on the other side. Once the wire is in the engine bay, connect the power wire to the battery or circuit breaker as described in Method 1.


The benefit of this method is that the wire is the most protected from the elements. The drawbacks are that it is the most time-consuming method of installation, it can be difficult to troubleshoot the wire, and you may need extra wire to reach the battery.





Tools Needed for Routing the Power Wire of a Trailer Wiring Harness


Additional tools for routing wires

Most of the wiring harnesses that require a direct connection to the battery, such as #119176KIT, come in a kit that includes wire ties for the Method 1 installation. If you choose Method 2 or 3, you may need some additional tools:



Updated by: Raymond P.

Last updated: 9/17/18





Questions and Comments about this Article

Joe S.

I'll be replacing my AGM battery with a lithium battery in my travel trailer very soon. I don't necessarily want the power wire to charge my battery. I just want to make sure my lithium won't draw a huge amperage across the small 12V wire trailer plug. Is this something I need to be concerned about?

Etrailer Expert
Reply from Kef G.

The change to a lithium battery shouldn't be an issue. It won't draw more than the 12V that your vehicle's wire is capable of sending. The 12V wire is mainly there to ensure that your battery doesn't completely die. Really, you'll end up with very similar results to your old battery in terms of charging from the 12V wire.

Stevec

Hi, I'm "finally" getting around to install your flat connector boat trailer wiring into my 2020 Tacoma - along with a 7 wire trailer plug in the bumper. I searched for best ways to actually connect my needed +12v wires (2 wires - one for the trailer converter box power and one for the +12 to the 7 way connector) - but Most battery post connection answers are generalized and in summary Just say "connect to battery +12v. I was just going to use ring connectors on my wires and attach to my battery clamp bolt .... But the ring terminals I can find are very thin and flimsy and I worry that they will deform on tightening and break due to any minor tugging on the cables. What is YOUR preferred method for attaching +12v wires directly to the + battery Post ??? Use a Special Post, Use just the flimsy rings, another type wire end connector ???? Much thanks and I do love etrailer !!

Etrailer Expert
Reply from Jon G.

Great question! We usually use ring terminals to connect to the battery, although I'm not sure how flimsy they are. The kit we normally recommend for doing an upgrade like what you're talking about is the wiring kit # ETBC7 . I've attached the instructions so you can see how we do it. You'll want to make sure that you use a 40A circuit breaker like part # PK54540 on that power wire as well to help protect both your trailer and your vehicle.

Ed F.

need info how to wire the2010 Caddy Sts trailer hitch wiring

Etrailer Expert
Reply from Jon G.

It looks like all of the trailer wiring harnesses that we have available for your 2010 Cadillac STS are universal options. This means that you'll need to do some testing of the wires behind your tail lights to get the proper connections. When you route the power wire to the front of the vehicle you'll want to stick near the frame and avoid any hot or moving parts. Unfortunately we don't have a specific video or installation guide for your STS but we do have a general purpose video of the Tekonsha Wiring # 119179KIT being installed which I am attaching for you to check out.

Alex

With all running light bulbs removed, I have 12volts available at each bulb socket. When I install any one of the bulbs, it does not illuminate and the 12volts goes away at each bulb socket. The bulb is a known good bulb. All bulb sockets are grounded to the frame, rather than at the mounting surface. Any ideas?

Etrailer Expert
Reply from Jon G.

Start off by checking all of your grounds. Make sure that they are secure and connected to a clean bare-metal surface as any rust or corrosion can be the cause of a bad ground. You can also try checking your vehicle and trailer connectors to make sure those don't have any debris or corrosion in them either. Here is a link to our helpful article about troubleshooting trailer wiring issues that you can check out as well.

Donnie

I have a flat connector with no extra wires what would I hook a power wire to .....I wired it to the same wires it was connected to but no power to the connector frame ground is good .

Etrailer Expert
Reply from Jon G.

If you have no power to the trailer connector wiring on your tow vehicle then you'll want to start off by making sure all of your fuses are installed and in good shape. If they look good then you'll want to move on to the wiring itself, making sure all of the connections are good and nothing is damaged.

Reply from Donnie

@JonG I ended up wiring my trailer harness to my rear light on truck but how do I get break light

Etrailer Expert
Reply from Jon G.

@Donnie Your trailer stop and turn signals need to be combined. If your tow vehicle has a separate brake circuit and your 4-Way harness only has 4 wires then you'll need to use a different harness. The Tekonsha # 119179 has a 5th wire for your brake circuit and then it has a ModuLite that combines the brake and turn signals to make it compatible with your trailer. You'll also need this wiring kit # 118151 to pull power from your tow vehicle battery. If you can let me know the year/make/model of your tow vehicle as well as what wiring you're currently using I can confirm whether or not you need to use these parts that I mentioned.

Reply from Donnie

@JonG hi thanks for getting back with me... I rewired on my truck all the lights are working except my brake lights checked fuses had to replace two but still have no break lights ... puzzled

Etrailer Expert
Reply from Jon G.

@Donnie What is the year/make/model of your truck and what wiring kit are you using?

Chris F.

Great explanation. Just one question: Is it better to connect the power cable to an "always hot" battery terminal or go through the fuse box and select a circuit that is only hot when the car is in "ACC ON" mode? I am asking because I would prefer not a have a circuit that always draws power running to the back, on the other hand, I am not sure if the electronics box at the hitch will be damaged by being switched on and off every time the car is started. Thanks.

Etrailer Expert
Reply from Jon G.

You will want to route the power directly to the vehicle battery so that you won't be adding any more draw to an OEM circuit on your tow vehicle. To turn the 12V power off when your ignition is off you can also install a Battery Isolation Solenoid # TR118665 which cuts power on that 12V wire with your vehicle off.

Reply from Chris F.

@JonG Thank you for the explanation, I did end up running it straight from the battery terminal. The box does not seem to draw a lot of power, so I will leave good enough alone.

Jerry C.

I have ran all new wiring and lights on my trailer, the turn signals work good and the brake lights work, but I cannot get the running lights(taillights) to work. What am I doing wrong?

Etrailer Expert
Reply from Jon G.

Have you verified that the running/tail light circuit is active on the tow vehicle? It could simply be that your tow vehicle has a bad fuse and you just need to look up in the owner's manual where that is to replace it.

Reply from Jerry C.

@JonG Thanks Jon, You are absolutely right, it was a burnt fuse. Thank you .

Rick C.

I want to upgrade from a 4 wire to a 7 wire connection and have an auxillary lead feeding my trailer as well as install a battery isolator solenoid and use with the Curt Echo Wireless Brake Controller. The existing 4 wire has connection to my battery. I know I need to run a second wire to my battery to power the wireless brake controller. But what about the auxillary lead? Do I need to run a 3rd wire to battery for that?

Etrailer Expert
Reply from Chris R.

When upgrading your 4-Way to a 7-Way (I recommend using the Wiring Kit # ETBC7L for this purpose) you'll just have to run that one black power wire up to the battery. This will activate the 12V circuit on the new 7-Way, which will both power the Curt Echo # C51180 and send power to the trailer battery to maintain its charge. The circuit still passes through the Echo so a separate power wire for that purpose isn't needed.

Reply from Rick C.

@ChrisR Thanks! Just to make sure. It's the aux power pin in the 1 oclock position right?

Etrailer Expert
Reply from Chris R.

@RickC You got it. If you're looking at the front of the 7-Way connector, the 12V/aux circuit is the pin in the 1 o'clock position.

Brian H.

I have a 2018 Toyota Tacoma, I installed the Hopkins 43354 harness, I’m not getting power to running lights for trailer, my brake lights and turn signals work fine , there is a relay for the running lights in fuse panel, will I need a bigger relay??

Etrailer Expert
Reply from Chris R.

You should not need a bigger relay. I would make sure the power wire has a secure connection the battery, the included fuse is installed, and the white ground wire is securely attached to a clean, bare metal surface. Those are typically what cause a certain light function not to work, especially the running lights which require more amperage.

Dennis L.

I have a 2018 Mazda CX5 Grand Touring with LED tail lights... The SUV does have a 12v 15amp power plug in the rear of the SUV already, Can the power connector get spliced/wired into this positive power source? Thanks

Etrailer Expert
Reply from Chris R.

Even though there's a 12V power source located at the rear of your CX5, I still have to recommend that you run the black wire from the Tekonsha # 118746 up to the vehicle's battery. This just provides the most reliable power source for the harness and doesn't risk any issues or damage with the factory circuit.

Ezequiel S.

I'm a little confused. Do I need TWO wires running from the battery or a single wire? I currently have 1 wire from the battery to the "module" (black box) but the 7 pole has a black wire for a 12v cable. I wonder how can I connect the 7 pole to the battery. Should I run a second wire? should I reroute the current wire going to the module and connect it to the 7 pole?

Etrailer Expert
Reply from Chris R.

To activate the 12V circuit on your 7-Way you'll need to run wiring from it up to the battery. I think the module you're referring to is a powered converter box for the harness, which is separate from the 7-Way 12V circuit.

J

Is it essential that I connect the hot wire I have a 2008 camry and have wired older models without a hot wire.

Etrailer Expert
Reply from Chris R.

All of the trailer wiring harnesses available for your Camry, including the Tekonsha # 118405 , do require a hot lead to be ran to the battery. As you've experienced, this isn't a requirement on all vehicles - but many, like your Camry, have either a more delicate factory wiring setup that needs protection or they simply don't provide enough power on the tail light circuits to power additional lights on a trailer. Because of this, I highly recommend running the hot lead when installing the harness. Otherwise you could risk damaging the factory wiring or not having enough juice for proper function of the trailer lights.

Larry K.

I am looking for how to run wire out of 5th wheel trailer. I already have truck completed. Do does the wire go from hitch into the battery compartment. it appears to go up into the fiberglass cover then in. But do I have to take off the fiberglass to run additional wiring? I am looking at adding a dc to dc charger from truck battery. Thanks,

Etrailer Expert
Reply from Chris R.

This will depend a lot on where exactly the battery is stored on your 5th wheel. You don't need to take the same exact route as your existing wiring, but you still need to find a way to go from the battery to the outside of the trailer. Take a look at the linked video below - I think it does a really nice job of showing the wiring process of a DC to DC charger on a 5th wheel like yours.

Kevin C.

Very good explanation. Thank you. No big intro and to the point. Excellent.

Etrailer Expert
Reply from Chris R.

I'm really glad we could help!



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