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

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? 81878

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. 67265

Reply from Rick C.

@ChrisR Thanks! Just to make sure. It's the aux power pin in the 1 oclock position right? 67339

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. 67507

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?? 81474

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. 67014

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 81092

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. 66746

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? 78630

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. 63813


Is it essential that I connect the hot wire I have a 2008 camry and have wired older models without a hot wire. 77220

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. 62910

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, 73751

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. 60124

Kevin C.

Very good explanation. Thank you. No big intro and to the point. Excellent. 72765

Reply from Chris R.

I'm really glad we could help! 59676



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