Wiring Trailer Lights with a 4-Way Plug (It's Easier Than You Think)

Many standard light-duty trailers, such as many boat trailers and utility trailers, use a 4-way wiring harness to connect to the tow vehicle. This connection provides the required running lights, turn signals, and brake lights for the trailer. There is also an additional pin provided for a ground wire.

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 or replacing corroded wiring is beyond us. We'll walk you through the trailer-wiring process for 4-way plugs below, so you can add or replace your wiring and get your trailer up and running in no time.

What is the Color Code for 4-Way Trailer Wiring?

Before we get into the step-by-step walkthrough, we'll first go over the standard color code for 4-way wiring harnesses. This will make it easier when it comes time to make your wiring connections.

The color code for wiring harnesses can vary between manufacturers, but the general standard for 4-way plugs is as follows:

Green: Right turn/brake light
Yellow: Left turn/brake light
Brown: Tail/running lights
White: Ground wire

What Size Wire Gauge is Used for a 4-Way Wiring Harness?

The minimum suggested wire size for a 4-way trailer plug is 18 gauge for the turn, brake, and running lights. The suggested minimum for the ground wire is 16 gauge.

NOTE: *Some manufacturers will use red/black wires instead of brown/green/yellow. For these harnesses, black is usually for the running lights, and red is for the turn signal and brake lights.

When making connections on your vehicle, 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 the 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.

Wiring a Trailer with a 4-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. We'll start with the trailer side first.

Trailer Side

Step 1: Prepare for Trailer Wiring Installation

Start by making sure you have everything you need to wire your trailer lights.

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 4-way harness.

Step 2: Connect Ground to Trailer Frame

Attach the white ground wire directly to a clean, bare section of the trailer frame using a self-tapping screw and ring terminal. For best performance, each component should have its own ground. For instance, each trailer light should be grounded separately along the trailer frame. This reduces the risk of a ground issue on one component impacting the whole wiring system. You can make these individual ground connections in step 4, when you connect the rest of your wires.

The trailer connector should extend 2-3 feet past the tongue. Make sure no paint, corrosion, or buildup is present at the ground location. (Clean if necessary.) If the trailer folds, make the ground connection behind the tongue, on the main body of the trailer frame.

NOTE: A bad ground is the most common source of issues when wiring a trailer. Check out our troubleshooting guide for help with common trailer wiring issues.

Wiring Diagram - Connecting Ground to Trailer Frame

Step 3: Run Wires Along Trailer Frame

Run the rest of the wires along the trailer frame to the taillights. Don't allow the wires to catch or chafe against any trailer components.

Feed your wires through the hollow trailer frame if possible for the best protection. You can also drill through the frame and use grommets if necessary. Alternatively, you can use wire clips or zip ties to secure loose wires along the frame.

Step 4: Make Trailer Connections

Once the wires have been run to their respective appropriate locations, it's time to make the connections to your trailer's lights.

Each light will have 3 wires: one that connects to the main harness's brown running light wire, one that connects to the turn/brake light wire (typically yellow or green), and a white ground that should be secured to the trailer frame.

Use a crimper to strip back the insulation on the wires. Then, use a butt connector and heat gun to connect the wires. Connect any additional ground wires from your lights.

HOW TO ATTACH WIRES (EXAMPLE):

Use a crimper to strip insulation from the end of each wire

Use butt connectors to connect your wires

Make your ground connection

Traditionally, the running light circuit (typically the brown wire) is carried on a single wire, so a jumper wire and some additional butt connectors are required to feed the running lights on the opposite side of the trailer.

As a simpler alternative, you can use a wishbone harness (also known as a Y harness) such as this one. A wishbone harness is a 4-pole connector with 5 wires that effectively splits the running light circuit into two wires. One wire runs along the driver's side of the trailer, and the other runs along the passenger side. Wishbone harnesses make the wiring job easier and eliminate the need for splicing in a jumper wire.

4-Way Wiring Plug Diagram Traditional Color Code

4-Way Wiring Plug Diagram Wishbone Color Code

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 your vehicle already has a 4-way connector, then great! Simply plug the trailer-end connector into the vehicle-end connector, and you're ready to roll.

If your vehicle does not have a 4-way connector, it's usually pretty simple to add one. Just keep in mind that wiring is a custom component. There are several ways that a wiring harness can connect to your vehicle, and the type of connector you use depends on what is available for your vehicle year, make, and model. You can use our fitguide to find custom wiring for your vehicle.

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.

Use circuit tester to confirm wire function

Step 2: Connect Ground to Vehicle Frame

Just like we did on the trailer, we now have to connect the ground on the vehicle side. Attach the white ground wire to a clean, bare metal surface on the vehicle frame. Apply some dielectric grease like # 11755 to help prevent corrosion.

Step 3: 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:

Some vehicles require you to route a power wire from the harness to the vehicle battery because the vehicle's electrical system cannot handle the amperage draw of the trailer lights. For a breakdown of how to route a power wire for a trailer wiring harness, click here.
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 4-pole connector itself—to help prevent corrosion.

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.

T-connectors can also be expanded to 5-way, 6-way, or 7-way trailer connectors through the use of a wiring adapter.

T-Connector connects to vehicle's existing wiring

A T-connector connects to your vehicle's existing wiring via an OEM plug

Clamp-On Style

Other harnesses, such as this one, clamp onto your vehicle's wiring without causing feedback, interference, or power draw from your vehicle's wiring system.

Clamp the harness's sensors to the appropriate vehicle wires, then run the hot lead to your vehicle's battery to provide the necessary power, since the harness does not draw from the wiring itself.

Pictured: Zero-contact interface sensor capable of reading current as it travels through wire

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.

Soldering Gun vs Butt Connector vs Quick Splice

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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Questions and Comments about this Article

Walt 10/30/2020

I have a Ford Transit 350 & a Wells Cargo trailer. The Wells Cargo trailer has an internal junction box where all the lights tie into, but I "believe" also has relays/switches in it. Is this necessary as don't most wiring go directly to the lights? Now does it matter that the trailer lights are LED & not incandescent? 102706

Reply from Jon G. 10/31/2020

It would be a little unique to see a bunch of fuses for your trailer wiring on the trailer side of things. Your vehicle will definitely have fuses dedicated to these functions though. So I wouldn't say it's necessary for standard trailer lighting (whether LED or Incandescent), but there really isn't any harm in it either - it would actually provide you with an extra layer of protection. It doesn't matter whether the lights are LED or incandescent, but we do recommend having them be the same type of light. 75547

Steve T. 10/3/2020

My old trailer connector has 5 wires into it with 4 pin connector. Bought a replacement flat pin connector. It is a 4 pin , but only has places for 4 wires.Replacing connector on truck side. 101773

Reply from Jon G. 10/5/2020

I've heard of this on a trailer before where the running light signal is run on 2 different wires so you can route that wire down both sides of your trailer, but I don't think I've ever heard of that for a vehicle-side connector before. The only time that something like this would exist is if there is a stop light that needs to be tied in, but even then the 5 wires would run through a module and wouldn't all be connected to the 4-Way. What is the year/make/model of your pickup, what functions do those 5 wires carry, and what connector did you purchase as a replacement? 75088

Beverly C. 8/29/2020

Replacing tail lights on 1977 Shasta but color wiring is 1 white 2red it had two bulbs I assumed top bulb was brake bottom two wires are running lights and turn signal but all I get is a dim flashing. Help 98043

Reply from Jon G. 9/3/2020

You need to test the wires that are on your 1977 Shasta and then connect them to the correct wires on your tail lights (I'm guessing you just replaced them). Typically white is used as a ground but this isn't a rule that's set in stone which is why you need to do some testing. 73078

Al E. 1/24/2020

Thanks for information. This answered my question! 64762

Al E. 1/24/2020

Change subject! Need to hookup tail lights on Ford Fusion to operate when being towed by Motorhome. 4 way flat harness 64756

Reply from Jacob H. 1/24/2020

If you have a Titanium Model you can use # RM-154 . This kit will wire into your factory tail lights, providing a nice clean look. If you have a Hybrid or Energi model you can use # C6304 . This will mount with strong magnets on your trunk and are designed to not scratch so long as the surface is clean. 54989

Helpful Links

Tow-Bar-Wiring/Ford/Fusion/2016/C6304.html

Tow-Bar-Wiring/Ford/Fusion/2016/RM-154.html