Wiper motor is wet internally and the grease in the gearbox has more than likely turned to a conductive paste messing up the park position wiper.

Maybe.

Or maybe the brushes are simply worn.

I'd have a ganders inside.

Mark - No Longer driving
Archie the Old English Sheep Mog...........
2010 Roadster 3.0 V6 (S3) wink