OK, the rear lighting is grounded by a single wire, black with a white tracer. The same ground lead feeds the EVAP purge, ISC, a few other things and the front lights. There's a connector just under the hump latch that is the rear light connector. I'd start there, since you haven't mentioned having other issues such as poor idle the issue should be isolated to the tail. To confirm you can use a jumper wire to connect the lights directly to the battery through the black/white tracer wire and see if that fixes your issue. If it does then you know what's wrong leaving the question of where.