Bad tail lights

The tail lights on my '95 Ranger keep dimming out. Sometimes they are full brighness, sometimes not on at all, sometimes somewhere in between. The brake lights and turn signals seem to be uneffected. Obviously it seems to be a grounding problem. I have not managed to get the problem to surface while I'm working on the truck however. My question is, where is where is the most likely place too start my search for the bad wiring?

If you think its a grounding problem check grounding locations. Usually the grounding locations are somewhere near the taillights (behind back tire mostly) and under the hood check tops of body panals. also check your taillight harness to make sure that its a solid connection.