7 Pin Trailer Wiring Harness | etrailer Install a 7-way, RV-style trailer connector on your vehicle with this easy-to-splice harnessLite draws power directly from the battery to avoid overloading your vehicle's circuits. Brake electrical system Includes harness with 4-pin connection point for plugging in a brake controller wiring adapter (sold separately) Supplies required fuses, wire taps, and additional hardware Constructed How to Wire a 4 Pin Trailer Connector to a 7 Pin Connector How to Wire a 4 Pin Trailer Connector to a 7 Pin Connector by Jack Hathcoat . This accounts for three of the colors in the wiring harness. The remaining color is the ground wire. Note the colors and their function. Step 2. Turn on the tow vehicle tail lights. Probe the seven-pin connector with a test light.