The theory is quite simple really - the fix somewhat less so. The fault is that one of your lamp units has a faulty earth - the connection to the bodywork. These faults usually occur in the tail lamp units - but which one is anyones guess. Believe it or not, a single filiament bulb fitted to a double filiament bulb holder can have the same effect, so go down this route if you've had any new bulbs recently.
John
March 2008