Not sure if this 'fix' will help with your problem but I had a rear wiper motor that would from time to time just stop working, either not beat at all or just stop halfway across the rear screen. It started happening the first winter I had the car and was really annoying (being OCD I can't stand to have things out of place and a rear wiper visible in my rear view mirror drove me crackers!). I had it checked out at the local dealers whilst in for service and they confirmed that the fix would be a replacement motor at a significant cost.
I managed to obtain a later revision of the motor from ebay and fitted it the following spring. All was well for a couple of days until we had a late cold spell and it started playing up again and finally stopped stuck up the rear screen (I was twitching at this point!). I then decided to check the fuse and rather than just do the visual check insitu, I removed it. I noticed that where the blades made contact with the fuse board there was signs of tracking. I turned the fuse over, reinserted and lo and behold the wiper worked, was faster than it ever had been and has been perfect ever since!
Obvious conclusion from this is the poor connection, coupled with cold weather was leading to voltage drop. Needless to say, I have a spare motor now...
Hope this helps someone and has to be worth checking/cleaning and reinserting the fuse before condeming/changing the motor!
