5 Wire Trailer Harness | etrailer This wishbone-style, 5-wire flat harness is great for a trailer with a surge coupler. The 40' long harness includes a 4' long ground wire and a blue auxiliary wire that signals your trailer Features: 5-Way flat connector provides a male plug for your trailer Works great for marine trailers with surge brakes 2 Tail light wires of wishbone-style harness make wiring your trailer more Amazon: 4 wire trailer harness Partsam Trailer Wire Extension Plug, 4 Pin Hitch Light Trailer Wiring Harness Extender with Rubber Cab for LED Brake Tailgate Light Bar, 32" Inch 4 Way Flat 4 Pin Universal Wiring Connector. by Partsam. $9 $ 9. 99. FREE Shipping. Only 20 left in stock - order soon.