Windshield wiper nightmare

CLK 320 Convertible: When the engine is running, the windshield wiper will not quit after I either push the washer button or turn it on by twisting the switch. However, they work normal with the key in the ignition and the engine off.

What I have done so far: Replaced the switch and also checked the fuses, wiper motor and motor parking sensor.

Could it be one of the relays? How would I go about testing them? Any other thoughts? Thanks for your help.

Sounds like your problem is in the windshield wiper motor.  Replacing it should fix your problem. The park switch is shorted internally.

Thanks for your reply.  Is there any way to check and / or fix this problem?  I would rather give it a shot than just spend the money on a new one without trying.  Any help would be appreciated. Thanks again!