Date registered: Dec 2003
Vehicle: MB 180 W120(1962) MB 200 W123(1979) MB C200 W203(2007)
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First of all check the wiring. There should be 3 wires (assuming it is a singlr speed motor). One is permanent earth. One is life when wiper switched on with ignition. These seem to be OK since your wiper works.
The third is permanent life when ignition on. This connects to the return to park mechanism inside the motor casing. If there is power, then you have to open up the motor. There should be a small plastic rod that makes 2 points come in contact for a short duration to activate the parking mechanism when you switch of the wiper.
The rod could be worn or broken.
You may have problem finding a replacement rod. After a long futile search, I used the plastic extension tube that comes with WD40. Cut two office pins to length so that they will not touch each other inside the but provide enough stiffness, the parking mechanism in my 1962 W120 (ponton) works fine.