My 2004 Mini Cooper S has developed an unusual problem. When I turn the rear wiper on to wipe, it runs a cycle of about 20 wipes and then stops at a different place on my rear window every time. Then I try and run the rear wiper again and it doesn't respond. I try the wiper repeatedly and sometimes it runs the 20 wipe cycle, but sometimes it doesn't, and it always stops in a different spot.

Any ideas what is going on?