not so sure its the switch, the wiper motor and rear wiper get a feed friom the same wires on the column. could be the wiring to the rear wiper motor or the relay. the relay you want is #72 on the fusebox, there is a linky in the mk3 how2 section which shows you where it is.
check the wiring in the rubber boot between the tailgate and car body, good place for splits.