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 signal to the hydraulic coupler to disengage the trailer brakes.
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 with most 4-pole trailer connectors.
