Rear wiper motors are prone to having a siezed shaft. This could result in the motor burning out or the plastic gears stripping or breaking.
If it is just the shaft that is siezed, you need to take the motor off - be very careful removing the plastic tailgate inside cover as the clips that hold it to the metal are stronger than the plastic cover material! I used a long flat plastic bar with a v cut in the end
Remove the cover and you will see the shaft with a small arm attached which connects to the gears. If this is broken you will need to look for a new motor. If you gently tap the (wiper) end of the shaft with a hammer (taking care not to damage the thread) so that it moves down a few mm. If it is solid spray copious amounts of Plus Gas in from the top and leave it for a few days. Repeat the spraying regularly.
If you can get the shaft to move up and down it will eventually turn. I am not sure if it comes all the way out as mine freed up nice enough for the wiper motor to work again.
The Sportwagon I have just bought has no rear wiper motor so I will be having a go stripping down an old spare one and will take photos this time!
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2