Any vehicle towing a trailer requires a trailer wiring harness to safely connect the taillights, turn signals, brake lights and other necessary electrical systems.

If your vehicle is not equipped with working trailer wiring, there are a number of different solutions to provide the perfect fit for your specific vehicle. Complete with a color coded trailer wiring diagram of each plug type, this guide walks through each available solution, including custom wiring, splice-in wiring and replacement wiring.

If you're looking to replace the wiring on your trailer, check out our trailer rewiring guide.

A. Custom wiring

Vehicle-specific plug-and-play harness that requires no splicing and provides a standard trailer connector

B. Splice-in wiring

Taillight converter that splices into your existing vehicle wiring and provides a standard trailer connector

C. Replacement plugs and sockets

Trailer plugs and vehicle sockets to splice in and replace damaged wiring connectors

Custom wiring is the ideal solution for installing trailer light wiring on your vehicle. A custom wiring harness or 'T-connector' is a vehicle-specific harness that plugs in without any spicing required and provides a standard connector output, such as a 4-way flat.

All CURT custom wiring comes with the exact components needed for a complete installation on the vehicle, including vehicle-specific plugs and an electrical converter, if needed.

A custom wiring harness has multiple plugs that are used to 'T' into the vehicle's taillight assembly, drawing power directly from the taillights or from a direct battery connection and providing a standard trailer light wiring connector. While custom wiring harnesses usually require two or more connection points, splicing and soldering are still not needed.

While some vehicles may not be equipped with a standard trailer wiring connector, they may have a special socket specifically intended for installing wiring, provided by the vehicle manufacturer. A custom wiring connector uses a single plug to plug into this factory socket and provide a standard trailer wiring connector.

Another type of custom wiring is original-equipment (OE) wiring or USCAR wiring. Select vehicles come with a standardized USCAR socket that provides a connection point for a CURT OE wiring harness.

Like a T-connector, an original equipment wiring harness plugs into the USCAR socket without any cutting, splicing or soldering required, and it provides a standard trailer wiring output, such as a 4-way flat or 7-way RV blade.

Learn more about USCAR wiring

If custom wiring is not available for your particular make and model, a taillight converter may be required to equip your vehicle with the proper trailer light wiring connection.

A taillight converter or electrical converter splices into your vehicle and provides a standard trailer plug wiring connector, typically a 4-way flat. The converter converts the vehicle's complex wiring system to be compatible with your trailer's simple wiring system. To learn more about vehicle wiring systems, check out our vehicle wiring systems guide.

If the trailer plug wiring on your vehicle or trailer is damaged or not working correctly, you can replace the connector with a CURT splice-in plug or socket.

Plugs (trailer side) and sockets (vehicle side) are available in all standard formats and can be spliced into your existing trailer plug wiring. Find the trailer light wiring diagram below that corresponds to your existing configuration. 

If you are rewiring your trailer completely, check out our trailer rewiring guide.

Following the standard method for wiring a trailer connector is vital to the safety of your vehicle while towing. Connecting the wrong color wires will result in mismatched taillight functions and confusion on the road.

Use this 4-pin wiring diagram to properly wire your 4-wire trailer plug.

• Green Right turn / brakes
• Yellow Left turn / brakes
• Brown Taillights
º White Ground

Note: The ground wire color for a 4-flat plug is white and should be properly grounded at the trailer tongue. Read the complete rewiring guide for more.

5-pin trailer wiring is very similar to 4-pin wiring, but it adds in a blue wire for the reverse or backup lights.

Not all trailers have reverse lights, so consider your own trailer as you wire in a 5-way plug.

• Blue Reverse lights
• Green Right turn / brakes
• Yellow Left turn / brakes
• Brown Taillights
º White Ground

6-pin trailer wiring introduces two new functions, a wire for connecting trailer brakes and a wire for +12-volt auxiliary power.

6-way wiring is most common on gooseneck trailers and allows for use with a brake controller.

• Brown Taillights
• Blue Electric brakes
• Green Right turn / brakes
• Yellow Left turn / brakes
º White Ground
• Black +12 volt

The 7-way round trailer plug is to be distinguished from 7-way RV blade plugs. The wiring connections and placement are different.

Be sure to review your own trailer connector before wiring.

º White Ground
• Brown Taillights
• Green Right turn / brakes
• Red Auxiliary power
• Yellow Left turn / brakes
• Black Reverse lights
• Blue Electric brakes

The SAE configuration of a 7-way RV blade plug should not be confused with the traditional configuration. Different wire colors are used for different functions.

Review your own trailer before wiring.

• Brown Taillights
• Yellow Left turn / brakes
º White Ground
• Blue Electric brakes
• Green Right turn / brakes
• Orange +12 volt
• Grey Reverse lights

7-pin trailer wiring is one of the most popular wiring configruations, particularly the traditional configuration versus the SAE J2863.

Use this 7-pin trailer wiring diagram to properly wire your 7-pin trailer plug.

• Green Taillights
• Red Left turn / brakes
º White Ground
• Blue Electric brakes
• Brown Right turn / brakes
• Black +12 volt
• Yellow Reverse lights

Note: Not all trailers are equipped with reverse lights (yellow wire). The position of this wire may vary for your own specific setup.

• Green Taillights
• Red Left turn / brakes
º White Ground
• Blue Electric brakes
• Brown Right turn / brakes
• Black +12 volt
• Yellow Reverse lights

• Brown Taillights
• Yellow Left turn / brakes
º White Ground
• Blue Electric brakes
• Green Right turn / brakes
• Orange +12 volt
• Grey Reverse lights

The traditional 7-way RV blade format is typically used on 5th wheel trailers, travel trailers and campers. The trailer wiring colors for this configuration are different than those on the SAE configuration.

The SAE J2863 7-way RV blade format is typically used on gooseneck trailers, utility trailers, cargo trailers and equipment trailers.

º White Ground
• Brown Taillights
• Green Right turn / brakes
• Red Auxiliary power
• Yellow Left turn / brakes
• Black Reverse lights
• Blue Electric brakes

• Brown Taillights
• Blue Electric brakes
• Green Right turn / brakes
• Yellow Left turn / brakes
º White Ground
• Black +12 volt

º White Ground
• Red +12 volts
• Blue Electric brakes
• Yellow Left turn / brakes
• Brown Taillights
• Green Right turn / brakes

• Blue Reverse lights
• Green Right turn / brakes
• Yellow Left turn / brakes
• Brown Taillights
º White Ground

• Green Right turn / brakes
º White Ground
• Yellow Left turn / brakes
• Brown Taillights

• Green Right turn / brakes
• Yellow Left turn / brakes
• Brown Taillights
º White Ground

Trailers are equipped with different plug types based on their electrical components. The chart below provides examples of common trailers and the types of plugs they typically use.