[tsubaki]

I can get into a whole speil of what it can be and still not cover the problem, but I'll throw some at you:
The primary cause of all trailer light *malfunctions is the lack (or damage) of a ground, the negative wire, (the black one on the battery) the white one at the trailer light wire harness, what ever color the manufacturer deemed correct in the wireing harness for that particular model that year. This is not to say you can derive a ground from the frame or metal part of a vehicle on newer vehicles, usually it must go thru the wire harness so as to not create other problems in the vehicle, especially ones with a body computer.