Your Guide to Trailer Wiring: How It Works, How to Wire, and More

You’ve probably wondered how drivers use turn signals and tail lights with trailers on the backs of their vehicles. Trailers typically come with tail lights and turn signals, drawing power from the vehicle they’re connected to via trailer wiring.

What is Trailer Wiring?

Trailer wiring is what’s used to power lights and other electrical accessories in trailers, including the tail lights, brake lights, turning signals, and much more.

These wirings are in different colors, each of which corresponds to a specific function.

Bear in mind that the location of wiring plugs and pins tends to vary depending on the vehicle. It’s always best to consult your vehicle owner’s manual before attempting to wire your towing vehicle.

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: If you connect a trailer directly to the vehicle wire harness, the added load from the trailer lights can damage the vehicle wiring in horrible ways. It has happened more than a few times to unsuspecting DIYers. The original switches and wiring aren’t sufficient to carry the extra load of trailer lights. Thus the OEMs will typically use relays for trailer tow packages.

7-Way Trailer Wiring for Passenger Cars

Here are the wiring positions for the seven pins that can be found on passenger cars.

1. Outside of the trunk behind the tail light housing on the driver’s side
2. Outside of the trunk behind the tail light housing on the passenger’s side
3. Inside of the trunk behind the tail light housing on the driver’s side
4. Inside of the trunk behind the tail light housing on the passenger’s side
5. Behind the rear bumper fascia on the driver’s side
6. Behind the rear bumper fascia on the center of the vehicle
7. Behind the rear bumper fascia on the passenger’s side

5-Way Trailer Wiring for Pickup Trucks

Here are the wiring positions for the five pins that can be found on pickup trucks.

1. Behind the tail light housing on the driver’s side
2. Behind the tail light housing on the passenger’s side
3. Behind the rear bumper on the driver’s side
4. Behind the rear bumper on the center of the vehicle
5. Behind the rear bumper on the passenger’s side

8-Way Trailer Wiring for SUVs and Vans

Here are the wiring positions for the eight pins that can be found on SUVs, minivans, and vans.

1. Behind the tail light housing on the driver’s side
2. Behind the tail light housing on the passenger’s side
3. Behind the rear access panel on the driver’s side
4. Behind the rear access panel on the passenger’s side
5. Behind the rear bumper on the driver’s side
6. Behind the rear bumper on the center of the vehicle
7. Behind the rear bumper on the passenger’s side
8. Underneath the rear floor panel

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Wiring Color Coding: How to Use a Trailer Wiring Harness

Most modern vehicles come with wiring harnesses, systems that combine various electrical cables and wires and organize them so they all fit under a single unit.

These cables all have different colors, each of which corresponds to different components. For example, green and yellow cables power the turn signals and the brake lights.

Some trailer wiring harnesses have 4-way functions, while others have as many as 7-way functions. The more functions a harness has, the more trailer features you can use.

Here’s a quick look at the different wiring harnesses and their wiring colors.

4-Way Function

The most common type of towing connector is the 4-way flat connector.

• Green: Right turn signal and brake light
• Yellow: Left turn signal and brake light
• Brown: Tail lights
• White: Ground

5-Way Function

5-way flat connectors offer everything 4-way flat connectors have, with the addition of backup light functionality.

• Green: Right turn signal and brake light
• Yellow: Left turn signal and brake light
• Brown: Tail lights
• White: Ground
• Red: Backup lights

6-Way Function

Trailers that come with both backup lights and electric brakes on top of other base features such as turn signals and brake lights use 6-way pole connectors.

• Green: Right turn signal and brake light
• Yellow: Left turn signal and brake light
• Brown: Tail lights
• White: Ground
• Red: Backup lights
• Blue: Electric brakes

7-Way Function

You’ll need a 7-way connection if you want to use backup lights, electric brakes, and auxiliary power on top of all the base features that come with 4-way flat connectors.

• Green: Right turn signal and brake light
• Yellow: Left turn signal and brake light
• Brown: Tail lights
• White: Ground
• Red: Backup lights
• Blue: Electric brakes
• Purple: Auxiliary power

How to Wire a 7-Way Trailer Plug

Unless you have the right tools and automotive know-how, it’s better to leave trailer wiring to a professional. However, if you’re confident you can wire a 7-way trailer plug on your own, here’s what the process usually involves.

What You’ll Need

• Philips head screwdriver
• Wire snippers
• Crimper
• Junction box
• Breakaway kit
• Brake controller

What to Do

Follow these steps carefully if you want to know how to wire trailer plugs safely and effectively. If you ever have a hard time with the pins and their locations, consult a 7-pin trailer wiring diagram.

1. Turn off all power sources in your vehicle to prevent electrical accidents while tinkering with the wires.
2. Prepare all seven wires for installation and pay close attention to their colors before making trailer connections.
3. Open your junction box and remove the nuts at the end of each junction box stud.
4. Cut excess wiring from your new cable and remove the insulation with a crimper.
5. Add ring terminals to the wiring using a crimper and match the colors of the wires from the trailer to the vehicle with the junction box.
6. Plug the trailer-end connector into the vehicle-end connector.
7. Apply a small amount of grease to all electrical connections, namely the plugs and the connectors, to prevent corrosion.
8. Turn on your vehicle and test the lights on your trailer to see if the wiring process worked.

Common Troubleshooting Issues

If you experience problems with the trailer lights after completing the wiring process, you might be dealing with short circuits, grounding problems, blown fuses, faulty connectors, and improper wiring.

• Short Circuits: Wires might be exposed and intertwined, causing short circuits.
• Grounding Problems: Ensure the white wire is properly connected to your vehicle’s ground.
• Blown Fuses: If the fuses are blown, electrical processes may become inoperative.
• Faulty Connectors: Ensure connectors are installed properly and are free of corrosion.

Improper Wiring: Confirm that all the wires match the correct function and color.