How To Wire It.com and Popular Mechanics have complete diagrams and color code charts for wiring a four-wire trailer. Additional information and diagrams are available on converting or adapting a six- or seven-wire vehicle harness to connect with a four-wire trailer.Continue Reading
Most trailers have three circuits; one for the running lights and one each for the left and right brake lights, with the fourth wire left for grounding. The turn signals are a function of the brake lights and are controlled by the flasher relay in the vehicle, which sends the correct signal to the appropriate filament. On a six-wire trailer the fifth and sixth wires are for trailers that have electric brakes and need an additional 12-volt power source. A seven-wire system uses the additional wire for an auxiliary power supply.
The vehicle to which a trailer is attached must have a matching plug or adapter. Many vehicles, particularly pickups and sport utility vehicles, are factory pre-wired for trailers. Some may even have a connector conveniently placed behind a spring-loaded cap. A new vehicle may have the harness needed in the glove box and simply require being plugged into the appropriate connector. O'Reilly Auto Parts sells adapters and temporary connection kits.Learn more about Vehicles