I assume you've done the obvious and checked the level of water in the resevoir? You could have a partially deteched or leaking hose on the rear motor. There are plenty of threads. Do a search. If it is leaking, it'll cause problems with all sorts of electrical bits (not just the wiper, door and lights, for example) as the water causes shorting and corrosion. There could also be a blockage.

Let us know how you get on.