Before you tow any trailer, you should ensure it has functional trailer lights. Because installation works related to electricity scare many vehicle owners away, they prefer the experts at trailer shops to do the job for them instead of trying to figure out how things work. However, in the simplest scenario on how to wire trailer lights, if you’ve bought a new vehicle, you only have to find a factory-installed connector on it and get the right harness to attach it to the trailer.
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Let’s see what types of connectors the trailer light wiring industry uses today. While the basic configuration is a 4-way flat connector with one female and three male ends, you may come across connectors with up to seven pins for additional functions requiring wiring, including electrically actuated brakes, power source for a winch, etc.
Basics: 4-Way Connector
This is the most common scenario. It has three poles for basic functions (running lights, turn signals, and brake lights) and one pin for the ground. This connector is commonly found on most light-duty trailers. On the vehicle side, there will be a female connector, while the trailer/RV harness will have a male connector. When wiring trailer lights, make sure to route the harness away from anything that could damage the wires.
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4-way connector: Trailer wiring color code explanation
- GREEN – Right Turn Lights/Brakes
- YELLOW – Left Turn Lights/Brakes
- BROWN – Tail Lights/Running Lights
- WHITE – Ground Wire
Basics: 5-Way Connector
Trailers longer than 15 feet and heavier than 1,500 lbs must have a brake system – that means another circuit for hydraulic brakes. The fifth wire (blue) is meant for reverse lights; this connection is needed to disconnect the hydraulic trailer coupler or actuator when the vehicle is backing up, thus deactivating the brakes on a trailer.
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5-way connector: Trailer wiring color code explanation
- GREEN – Right Turn Lights/Brakes
- YELLOW – Left Turn Lights/Brakes
- BROWN – Tail Lights/Running Lights
- WHITE – Ground Wire
- BLUE – Reverse lights
Basics: 6-Way Connector
Apart from providing basic functions, this connector has 2 more ports for electric brake control (blue) and 12V power supply (black or red).
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6-way connector: Trailer wiring color code explanation
- GREEN – Right Turn Lights/Brakes
- YELLOW – Left Turn Lights/Brakes
- BROWN – Tail Lights/Running Lights
- WHITE – Ground Wire
- BLUE – Reverse lights
- BLACK – 12V power supply
Basics: 7-Way Connector
7-way connectors allow for all functions provided by 6-way connectors (three basic lighting functions, electric brakes, and additional power supply), with the seventh wire for backup lights. Basically, it is another 12V circuit typically used for a reverse light / reverse lockout for trailer brakes.
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7-way connector: Trailer wiring color code explanation
- GREEN – Right Turn Lights/Brakes
- YELLOW – Left Turn Lights/Brakes
- BROWN – Tail Lights/Running Lights
- WHITE – Ground Wire
- BLUE – Reverse lights
- BLACK – 12V power supply
- PURPLE – backup/reverse lights
Truck Trailer Light Wiring: Options & Packages
Now that you know the types of connectors, you have to determine what you have on your vehicle to make the connection to a trailer. Many modern tow-capable trucks and SUVs, especially those by the Big Three, come equipped with trailer packages from the factory as part of the USCAR program. This organization (United States Council for Automotive Research) united three major American automakers (Chrysler, Ford, and GM) in their intention to advance automotive technologies. They developed a universal trailer connector that has been used on their vehicles since the 1990s. To determine how to wire up trailer lights, you need to know whether your vehicle has a factory-installed trailer package and whether a plug-and-play T-connector is available for it on the market.
- If your vehicle came prewired for towing, it may have a factory trailer harness connection (USCAR connector) in the rear bumper. This is the best case scenario, you’ll only need to buy the appropriate wiring harness with the right connectors on each end.
- If your late model car, truck, or SUV does not have a trailer package from the factory, you may go with a T-connector made specifically for your make and model. This vehicle-to-trailer wiring harness creates a standard trailer wiring connection by plugging into vehicle harness and tail light assembly, and then routing the 4-way harness to the rear end of a vehicle.
- If you own an older vehicle, chances are it will have neither trailer package factory installed nor T-connectors available for it. In this case, you can go with one of universal trailer wiring kits the aftermarket offers today. Reliable and relatively easy to install, these kits contain everything necessary to make the connection in an hour or so.
As a matter of fact, the aftermarket offers harnesses to join two connectors of any type. Even if your vehicle is not equipped with a connector, it may have a wiring plug located in the rear. Depending on the model, the location may vary from inside of trunk to under the rear floor panel. You may need to inspect your vehicle thoroughly or contact the manufacturer’s customer support to ask whether your vehicle is equipped with it and find out its location. If there are absolutely no provisions for trailer lights, you are electrically inclined or have a rough idea of how to wire trailer lights, you might consider splicing into your existing wiring. Fortunately, almost all wires you need (except for a brake controller wire) connect to your vehicle’s tail light assembly. In this case, you will need a set of wiring taps and a pair of pliers.
Trailer Wiring Diagram
To connect the electric system of your trailer to the vehicle, you will be using special connector. Above we have describes the main types of trailer wiring diagrams. Below is the generic schematic of how the wiring goes.
4 pin trailer wiring diagram
This type of connector is normally found on UTVs, ATVs and trailers that do not have their own braking system. Note, that this type of 4-pin connector is less common, that 4-pin flat connector. As a rule, you can find these connectors on the older trailers and older vehicles built in the U.S. At the moment, neiter tow vehicles nor trailers are equipped with round 4-pin connectors from the factory.
- commonly sold without the wires;
- complete kits are available (see product description);
- made from die-cast metal construction;
- offer a concealed terminal with a dust cover for protection against the elements.
5 wire trailer wiring diagram
5 way trailer wiring diagram allows basic hookup of the trailer and allows using 3 main lighting functions and 1 extra function that depends on the vehicle:
- daytime running lights;
- turn signals;
- brake lights;
- an extra pin allows using another extra function.
As a rule, 5-Way flat connectors are used for trailers that feature surge brakes or hydraulic brakes. The extra wire, as a rule, is used to power backup lights. When it is plugged, it disengages hydraulic trailer actuator when you reverse, so the trailer brakes are off at that moment.
Trailer wiring diagram 6 pin (round)
6-Way trailer connectors are used in the following vehicle applications:
- gooseneck trailers;
- utility trailers;
- boat trailers;
- 5th wheels.
6 way wiring diagram is explained in the schematic and table below:
| Wire color | Gauge | Sealed Connector Terminal | Zinc connector Terminal |
|---|---|---|---|
| White | 10/14 | Common Ground | GD |
| Blue | 10/14 | Electric Brake | S |
| Green | 14 | Right Stop & Turn | TM - Tail & License |
| Red | 10/14 | Auxiliary | LT - Left Stop & Turn |
| Brown | 14 | Tail & License | RT - Right Stop & Turn |
| Yellow | 14 | Left Stop & Turn | A - Auxiliary |
7 pin trailer wiring diagram with brakes
7-Way trailer connectors are used by the following vehicle types:
- on most recreational vehicles;
- on larger gooseneck;
- large 5th wheel trailers;
- large boat trailers;
- full-size utility trailers.
7 way trailer wiring diagram is explained in details in the picture and the table below:
| Wire color | Gauge | Sealed Connector Terminal | Thermoplastic Connector Terminal |
|---|---|---|---|
| White | 10 | Common Ground | Common Ground |
| Blue | 12 | Electric Brake | Electric Brake |
| Green | 14 | Tail & License | Tail & License |
| Black | 10 | Battery Charge | Battery Charge |
| Red | 14 | Left Stop & Turn | Left Stop & Turn |
| Brown | 14 | Right Stop & Turn | Right Stop & Turn |
| Yellow | 14 | Center Auxiliary | Center Auxiliary |
Trailer Wiring Reviews
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Thank you, I've been watching videos for 3 days and found plenty on a four pin connector but none on adapting a seven pin adaptor to a boat trailer with surge brakes. Plenty of talk but you guys have cleared up my issues completely. Thank you so much, you just saved me over three hundred dollars. That was the cheapest estimate I received from the so called pros. Uhaul wouldn't even give me a price, Pep Boys didn't have a clue what I was asking and the boat shop wanted $500 and the trailer place wanted $300.
Thanks again
Being from Australia trying to grasp how the turn and brake lights are on the same wire. This still evades me, is it that if the turn is flashing on/off,the brake light does not activate, but if hard on it does?
Seems very confusing since often when turning, you are also on the brakes. Any info for clarity would be great.
Thanks.
very good diagrams and explanations of various 4-7 pin connectors.
very helpful in tracing my running lights
Thank you for making it all makes sense!
there can be same confusion between your wiring description(table), and diagram. in my opinion the description, on the diagram, should be the same as the description on the table. for example, the break terminal should say "electric brake, blue, 12 gage wire. all terminals should be described in this format.
A very good explanation what the black wire in the 7-way connector is for. I couldn't find the explanation in 4 more sites I reviewed. Thanks for the help!
Thanks for the clear explanation of the trailer wiring schemes. You guys helped me a lot.
