I'm having the same problem. My rear passenger lights (brake, turn and back up) go out while driving. At first, it seemed like a fluke, then I parked the car at work, and the problem went away when I drove home from work. A few days later, it happened again. I replaced all 4 bulbs (turn, stop, brake, and rear marker) and no effect. Oddly enough, I slapped the rear fixture out of frustration, and it worked...for a few days at least. I checked the flat connector by the rear passenger lights in the battery compartment, and it seems secure.
So somewhere in the rear passenger lighting fixture, something is a bit loose, what what? Any ideas?