98 Dodge Truck taillight wiring problem

I have a '98 Dodge truck here with a taillight wiring issue.  When you hit the brake lights, the backup light illuminate.  The backup lights also illuminate opposite to the blinkers, when the parking lights are on (ie. the blinker will flash normal, and when not lit, the backup lights will be lit).  When the backup lights are illuminated incorrectly, they aren't as bright as when normally lit.

This truck is loaded with aftermarket accessories, none of which were installed here.  I know this is going to be like finding a needle in a haystack.  Just wondering if Dodge had any quality issues in the lighting department to know about.

Just wanted to let everyone know that I solved the problem.  The first thing I checked in this situation was the grounds just before the bulbs.  I was running extra ground jumpers, one at a time, to just before the taillight bulbs.  That did not show the problem.  I spent another half-hour checking the trailer wiring and the wiring to the front of the truck.  Only when I grounded all the bulbs with extra jumpers did I find the problem.  So, I basically soldered in extra ground wires to a cental grounding location to fix the problem.