did you check the voltage coming from the truck? it sounds like your ground isnt good on it. i dont believe the trucks like yours and mine come with trailer connectors from the factory they just run the wires for you so what you got was probably added by someone before. anyway the connections between the aftermarket connector and factory wires corrode especially if butt splice connectors are used. if thats the case you should get a new trailer pigtail and solder and heatshrink it in.