Normally caused by a bad earth connection in the tail light wiring. does it happen on both sides or just one ? remove light clusters usually held in by plastic clips and come out as one piece complete with bulbs. Look for the earth connections to vehicle body remove and clean back to bear metal. check wiring loom connections into light units themselves look for dirty or corroded terminals if everthing's clean should work ok. indicator is finding a partial earth through tail light circuit.
Tony, February 2009
