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 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:

A junction box to organize and secure wiring
A breakaway kit in case your trailer becomes disconnected
A brake controller to control trailer braking

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.

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.

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 BOX

If 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 - Strip Wire Insulation

WITHOUT JUNCTION BOX

If 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.

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 Installation

IF YOU HAVE A 7-WAY CONNECTOR

If 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 CONNECTOR

If 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 CONNECTOR

If 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.

If you have a 4-way plug, add a 7-way with a 4-to-7-way adapter

Use a circuit tester to confirm wire function

Step 2: Make Vehicle Connections

Your 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 ADAPTERS

An 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 STYLE

Some 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 STYLE

Hardwire 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.

When Finished

After 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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Billy V.

11/4/2023

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.

10/8/2023

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?

Mike L.

10/10/2023

@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.

Helpful Links

faq-wiring.aspx

Wilson J.

9/25/2023

I have an older motorhome. I want to use the 7 pin to tow a car dolly with an electric brake. When I checked the 7 pin on the vehicle (Curtis tester) all worked but when the brake was applied the 2 turn signal lights on the tester were on and the brake was not. Any ideas?? Help!

Ronald D.

7/27/2023

I have a set of LED tail lights that I want to put on my old old... OLD caravan. Had a quick attempt before I left on holiday, but everything I tried was a bit of a disaster. Lights flashed and there was actually sound as well. We have had problem for a while each time I connect caravan with the tailgate brakelight flashing. After we hold the brake pedal, this stopped and I always assumed it was holding current somewhere because of Redarc brake controller. But this problem is keeping me baffled. Have always done my own wiring, often running heavier wire than required. Done this for many years and want to change all the lights on the old girl once I get my head around what is happening. See there are units that add resistance to a light but how do you know which one to add. My friend has unit on his drawbar that has the standard wires enter an aftermarket box. We were going to see what is in box because it did same thing. The box is actually sealed shut with gland nuts on each end. I am sure I could do a similar unit if I knew what they have done. Any ideas would be appreciated.

Mike L.

8/3/2023

@RonaldD The problems you're experiencing are occurring because LED lamps draw only a fraction of the power that incandescent lamps do. To fix this, you'll want to install load resistors on the turn signal circuits. We offer them as part # P230004C-2.

Hcar

12/10/2022

I am wire from go on a 2013 trailer, enclosed 16’. From reading your comment to another regarding relays, My assumption would be 1 relay each, left turn, right turn, running lights, reverse lights, and a general one for any switched LEDS; OR JUST THE FIRST FOUR:?

Mike L.

12/16/2022

I'm not sure if I'm understanding what you're trying to do here... Are your LED trailer taillights hyperflashing because the tow vehicle has bulb-out sensors? if that's the case. you'll need to install load resistors like # DI34ZR on the right and left turn signal circuits. If that's not what you've got going on, let me know!