The symptoms you're describing are classic grounding troubles. If one or both light assemblies don't have a good, low-resistance ground the voltage seeks another path. This will cause lights to burn brighter, dimmer, or even quit working at all, depending on which bulb filament is trying to burn, and what other circuits happen to be energized at the time.
Start out by installing a relatively heavy wire between the bed sheet metal and the frame, a #16 or bigger. Scrape the body metal and the frame down to bare, bright metal, or use an angle grinder or a sanding disc to buff away any paint, primer of rust. Once the ground wire is bolted on solidly, you can prime and/or paint over the connector to prevent rust. Do the same thing with the light assembly- - - -don't rely on the attaching bolts to provide a ground- - -run a ground wire from the light to the bed metal.
I'll bet your problems disappear!
Jerry
