How to fix a rear wiper motor that stutters or doesn't return.
The problem is too much grease in the motor housing.
First remove the motor from the car, then unclip the plastic cover over the electronic controls on the back of the unit. It'll look like this when its off.
Check the joints between the power connectors and the circuit board are okay, unlikely to cause the fault, but possible.
Next, remove the white thing with the circuit board in from the motor. It slids to the side after a clip is released.
The white plastic block this has three prongs on the back. These control the return of the wiper to park position. They look like this
I took the pics after cleaning everything up, but the metal wheel inside the motor was covered in grease as where the metal prong contacts. I cleaned off the grease with a bit of rag and put it all back together. It works fine now.