Troubleshooting Trailer Wiring Connections and Confirming Solid Ground Wiring for Proper Function  

Put new lights on trailer, 4 pin, has full length ground wire Truck has 4 pin connector, no factory wiring, but has a an aftermarket t like connecter that is plugged in between wiring and the tail lights, the ground is part of the t connecter. And is not grounded to the frame. On trailer, all the lights work as they should until the following When you turn on the head lights the turn signal lights when engaged flash on both sides. When you turn on the head lights, and you put the brake on, the running lights go off. The harness on trailer has a full ground wire white that all the lights are attached to. A harness tester, shows the wiring on truck is fine. I have metered and tested all trailer wires for continuity, which is fine. I tested all trailer wires and lights to see if their was any signal bleed across the wires, their was none. I did the same on truck, there was none. I also cleaned all connector plugs and put di-electric grease on same. What now? Thanks

asked by: Joe D