this may sound silly.... but.... have your alternator checked.
Are you experiencing "any" other electrical gremlins? Losing light bulbs more frequently than usual? Radio acting strange...(speakers cutting in and out.. due to amps turning off)

I had a flakey voltage regulator that was allowing slight surges through the system... It would trip my radio amps, cause my tail lights to fail prematurely, and cause a phantom wiper symptom as you described...

it might just be a bad stalk on the column.... but if you are seeing any other odd things, I'd suggest a voltage check... it doesn't take much to cause a lot of crazy in those cars. Good luck

__________________
Jeff
'87 300E
'92 400E
'06 ML350