check that the motor has definitely gone. There is a common problem with these cars. That is that the wiper blade stops half way along the windscreen. This is because the base of the wiper blade moves up and down a shaft which needs to be regularly greased. Simply prise off (carefully) the disc shaped cover at the base of the wiper and you will see a chrome shaft, grease this. Worked for me twice and yet i have seen some mercedes rip off garages charge for a whole new motor and charging 500 quid. hope this helps.
Biscuits, July 2008