HomeTips & GuidesHooking Up A Wire Harness On A Car For A Trailer

Hooking Up A Wire Harness On A Car For A Trailer

About Hooking Up A Wire Harness On A Car For A Trailer: A trailer could serve several uses, depending on your wants, including transporting camping utilities, boats, materials, and goods. 

The tricky thing involving trailers is connecting one to your primary car. 

Color coding is the most important thing to know if you’re wondering how to hook up a wire harness on a car for a trailer

From there, the installation should not be too complicated.

Table of Contents

Trailer Wiring Harness Understanding

Trailers are separate extension vehicles you need to link to your primary vehicle.

You can add a trailer wiring harness connector to the rear of your primary vehicle to connect your trailer, thus having the indicator and brake lights working properly.

Different connector types from 4 to 7 pins enable power transfer for lighting and auxiliary functions like backup lights, power supply for interior lights or a winch, and electric trailer brake control.

Therefore, choosing a connector with the right pins is important to enjoy your trailer’s features. 

Trailer wiring uses various colors to make it easier to understand and correctly link your wires with your connector and primary vehicle.

FunctionColorMin Wire GaugeWhere to attach (Vehicle side)Where to attach (Trailer side)
7-WAY6-WAY5-WAY4-WAYRight TurnGreen1816Car Wiring Harness (right turn)Right turn signal of the trailer
Left TurnYellow1816Car Wiring Harness (left turn)Left turn signal of the trailer
GroundWhite1612Car Ground Point (rustproof, uncoated, metal)Ground point of the trailer (rustproof, uncoated, metal)
Marker/ TailBrown1816Car Wiring Harness (taillight)Taillights
BrakeBlue1812Electric brake control, power for brakesBreakaway switch
BatteryBlack or Red12Fused battery lead or fuse blockInterior lights, battery charger, breakaway kits
BackupPurple16Car Wiring Harness (backup circuit)Hydraulic coupler/ backup lights

Trailer Wiring Harnesses Types and Different Plug Types

Having discussed the different colors of wires and their uses, you might still not be sure why there are various types of trailer wiring. This is because there are various towing and trailer scenarios, each with specific uses and functionalities.

4-Way configuration

The 4-way configuration has four wires: green (brake/right turn), yellow (brake/left turn), brown (running lights/taillights), and white (ground).

4-ways are available in round or flat configurations. Smaller trailers often use flat configurations, while industrial applications use round configurations.

5-Way Flat

The 5-way configuration has 5 wires: white (ground), green (brake/right turn), yellow (brake/left turn), brown (taillights), and blue (trailer brakes).

6-Way configuration

The 6-way configuration has 6 wires: white (ground), green (brake right turn), yellow (brake/left turn), brown (taillights), blue (trailer brakes), and black (+12 V).

6-ways are available in square and round complications. Often, the square configuration is utilized in popup cameras.

7-Way Round

The 7-way configuration has 7 wires: white (ground), green (brake/right turn), yellow (brake/left turn), brown (taillights), blue (trailer brakes), black (reverse lights), and red (auxiliary power).

7-Way RV Blade – SAE Configuration

The 7-way SAE configuration has similar wiring to the 7-way but has 2 different wire colors: white(ground), yellow (brake/left turn), green (brake/right turn), blue (trailer brakes), orange (+12V), and gray (reverse lights).

7-Way RV Blade – Traditional Configuration

This configuration has the number and color of wires as the 7-way configuration, but the wires have completely different functions.

White (ground), yellow (reverse lights), green (taillights), brown (brakes/right turn), blue (trailer brakes), black (+12 V), and red (auxiliary power).

7-Way Connector

Hooking Up A Wire Harness On A Car For A Trailer: 7-Way Connector

Mounting Your Trailer Wiring Harness Connector

Before doing any work on your car’s wiring, go through the manual and thoroughly inspect if you already have a connector installed.  If your car doesn’t have an existing wiring harness, it probably has the necessary plugs to enable you to install trailer wiring.

Towing Package and Trailer Harness Wiring Pre-Installed

Your car probably already has a car-side 7-way RV blade or 4-way flat connector. 

Normally, a 7-way connector should be close to the center of the rear bumper, mounted on the bumper itself or a tab.

With a 4-way connector, you can expect to find it tucked under your car’s rear. 

When you must tow, pull out the connector and close the back door or trunk. A rubber weather strip provides a seal for the door to prevent wires from getting pinched.

If the wiring on your car is not the same as the one on the tow, you can use a wiring plug adapter/T-one connector to make the two different configurations compatible.  Also, you don’t have to splice into your car’s wiring system using an adapter.

However, you can’t adapt upwards, for instance, a 4-flat on your car to a 7-blade on your tow. But it’s possible to adapt downwards from a 7-flat on your car to a 4-blade on your tow.

The adapter normally uses a 4-pole trailer connector, which can easily adapt to match bigger 5-pole and 6-pole styles. 

An adapter can plug into flat 4-pole connectors and has wires for additional functions like auxiliary power for tools or a winch, providing power for trailer brakes, power lead for utility lights, and reverse.

If your trailer or car has something besides a 4-way connector, like a bigger 6-way plug, ensure you find the right adapter for your trailer and car. 

Hooking Up A Wire Harness On A Car For A Trailer: Towing Package That Has OEM Sockets

For such situations, you’ll require a custom wiring harness connector to plug in the OEM sockets.

You’ll need to find your socket to check the type of connection to determine the trailer wiring harness you’ll need.

Hooking Up A Wire Harness On A Car For A Trailer: No OEM Socket or Towing Package

If your car doesn’t have OEM sockets or a towing package, you must get a custom wiring harness configuring the taillight connections.

Check the trailer wiring harness close to the coupler or tongue to determine the type of wiring connectors installed. 

No OEM Sockets, Towing Package, or Custom Harness Available

For such a situation, you’ll need a taillight converter to splice into your car’s existing wiring. 


Remember, getting the right connector is important when mounting your trailer wiring harness. For all your trailer wiring harness needs, contact Cloom Tech.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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