Boat Trailer Wiring Harness | etrailer The 5 way flat trailer connector is most commonly found on boat trailers that have hydraulic brakes. The fifth wire is hooked up to the reverse light circuit on the vehicle which then sends a signales. 5-Flat electrical wiring connector Fifth wire added for surge brake lockout circuit which is becoming more common on today's newer boat trailers. 5-Flat wiring Connector will still work Amazon: boat trailer light wiring Amazon: boat trailer light wiring. Skip to main content. From The Community. SUZCO 25 Foot 4 Wire 4-Flat Trailer Light Wiring Harness Extension Kit, 4-Way Plug 4 Pin Male & Female Extension Connector & Wishbone-Style with 18 Gauge White Ground Wire with SAE J1128. by SUZCO.