Hopkins 50928 Tow Doctor Trailer End Wire Harness Test Unit Designed so that one person can make sure a trailer has been wired properly and all functions are working Tests all trailer functions - left turn, brake, tail, backup lights, electric brakes, and auxiliary power Trailer lights illuminate and LED lights on the tester show function operation Adapts to 4 wire flat, 6 pole round, or 7 RV blade For electric brakes, magnets are activated to determine correct operation