Trailer Wiring Diagrams | etrailer A quick splice slid through the yellow wire on the trailer connector and around the just found vehicle wire will clamp down to provide a secure connection. The three types of hardwire kits are: Standard 4-Pole Wiring Harness - For use with vehicles that have adequate power and standard wiring system, these simply connect into existing wires on the vehicle and have a 4-pole flat connector to attach a trailer.