re: Looking for help with an issue with my boat trailer lights

Ground. Its always the ground. All trailer light issues are a grounding issue....almost.

If it ain't a ground the only thing to do is use a multimeter at the truck and make sure the male end is functioning. If it is ring the trailer wires out although if they worked before and nothing has been done to them it is a ground. Always a ground.

The vehicle specific harnesses that most vehicles use today work great but in my experience when they fail they do so catastrophically without any indication they are going to do so. If it is done the old fashioned way by cutting the vehicle wires and twisting them together and taping you could have a bad joint. if it was done with the damned blue crimp on taps they also fail pretty regularly....but if it is a realatively new vehicle it probably has the plug in harness and in my experience they don't go bad they just fail compeletely with no symptoms...