So what you are saying (in my inexperienced mind) is that if the ground is bad, it can cause one light to be bright and one dim because the affected side will back up and find a different electrical route (ground).
Jim,
Did you ever seen the lights working correctly? You stated that some of the truck was retired, are the lights included with it? Just a thought, Have you put a meter on the wires yet to ensure that the previous owner had it correctly wired or flipped the wires and retested?
Ground or flipped wires is a cheap/easy fix. I am curious as to what the diagnosis is.
Chris
