Trailer Wire Harness and Connector - Best Replacement Get the job done with the right part, at the right price. Find our best fitting trailer wire harness and connectors for your vehicle and enjoy free next day delivery or same day pickup at a store near you! How to Install Trailer Wiring | Color Coded Diagrams Option A: Custom Wiring Installation. Custom wiring is the ideal solution for installing a trailer wiring harness on your vehicle. A custom wiring harness or 'T-connector' is a vehicle-specific harness that plugs in without any spicing required and provides a standard connector output, such as a 4-way flat.