The basic components vary depending on the trailer type being towed. However, trailer wires are color coded, with the colors universally applied and understood in the towing world. The number of wires used ranges from four to seven.

The lightest duty trailers use a four-wire system: green, yellow, white and brown. Green wiring attaches to a trailer’s right turn signal and brake lights; yellow wiring to the left turn signal; white wire to the ground point and brown wire to the taillights. The types of trailers that use this 4-wire system include pop-ups, utility tows and small trailers.

Trailers with built-in brakes use a fifth wiring that’s blue and attaches to the brakes.

Trailers that use a sixth wiring that’s red and a seventh wiring that’s purple include larger trailers, horse trailers and tiny houses. The red wire connects to an external charging battery and the purple wiring connects to the backup lights.

As noted in the previous answer, burned out light bulbs or a poor grounding connection are common problems with trailer wiring. You can test light bulbs with the help of an assistant who can stand behind the trailer and watch as you perform functions to see which bulbs light up as they should, and which don’t. Once you replace any burnt out bulbs, check the ground wire as noted above, and from there check the tow vehicle connector; check to see whether a fuse has been blown and then check all wire connections with a vehicle light tester. Then check for damaged wires that need replacement. As a last resort, take your trailer to a professional for a thorough inspection, diagnosis and repair.

4-pin trailer connects your vehicle with your trailer to provide it with lights for right and left turn signals, brakes and running lights plus a ground wire connection. A 7-pin connects your vehicle with your trailer for turn signals, brakes, reverse and running lights plus a connection for an external charging battery and a ground wire.