Check the earth connection(s) from the rear light clusters and on the clusters themselves particularly i fthese are based on a printed circuit board - these can develop cracked tracks.
X, July 2015
forgot to add that if a stop and tail bulb is changed ,it is fitted the right way [offset pins!]
k, July 2015
sounds like a wrong bulb fitted in rear light bulb board assy.check handbook for correct bulb/position
