Bizarre electrical situations like these are nearly always caused by one or more of the lamp units having a faulty earth - this is where all of the bulb holders are connected to the cars metal bodywork. An auto electrician would run an extra cable to each bulb cluster to try to identify which one was at fault. Another possible cause is the incorrect indicator or tail lamp bulb being fitted accidentally - a 2 filiament bulb into a single filiament holder, for example.
John, December 2007
