Trailer Wiring Harness Kit | etrailer For vehicles with LED tail lights, this harness adapts your tow vehicle's 3-wire, separate tail light system to the trailer's 2-wire, combined system.Kit includes 4-way flat connector be required to send more power through the wiring harness to fully light the trailer's signals. Universal Fit. Trailer Hitch Wiring. Trailer Connectors. Hopkins. 4 Flat Amazon: trailer light harness kit ROSI 6ft 4 Wire 4-Flat Trailer Light Wiring Harness Kit 4 Way Trailer Wire Extension & Wishbone-Style with 20 Gauge White Ground Wire SAE J1128 Rated，with 4 Flat Connector for Under 80" Wide Trailers. $11 $ 11. 49. Get it as soon as Tue, Apr 30. FREE Shipping on orders over $25 shipped by Amazon.