Rear wiper A4 Avant (B5)

Every year, my girlfriend's A4's flaw comes back. Now it's the following: when you turn on the tail wiper, it works. Only he no longer wipes in the interval but constantly. He also wipes the whole time further if he should be switched off according to the steering stick lever and also stops in the middle of the disc. What could that be: steering pole lever, relay or motor. I read what the washing water hose is going through the engine and because it is spinning somewhere and puts the engine under water. Can such a thing trigger such causes?