Fitted RS components device and tested - working perfectly !
So - if the rear window motor fails to operate from the CCU - i.e. when you tell it to, then you can probably fix it for £1.48p (free postage) !
Here's how:
Remove rear window regulator and motor complete from rear door.
Unscrew Torx screws and remove motor unit complete with drive pulley from the assembly.
Now - to check that the motor itself is working (and if it isn't you've probably found the problem) connect 12V to pin 3 and 6 on the multiplug on the motor unit.
If it runs - carry on reading, if it doesn't get a new motor !
Carefully remove the 4 torx screws holding the motor body to the pulley etc.
Motor removed from pulley assembly.
Carefully slide off the collar (black) which contains the brushes (don't damage the brushes as they 'pop' over the black ring and off the commutator) and the hall effect device (likely reason for failure).
Observe the small black transistory looking thing with three legs. It is slid into a plastic mount and the legs soldered.
Here is said device - it is a 'hall-effect' sensor - function described later.
Order yourself a seemingly equivalent device from RS components - no need for account or log in and post free so it gets my business !
Part number SS40A - Honeywell digital hall effect sensor - RS Stock Number 822-3778.
Pay £1.48 inc postage.
The pin out is identical to the old device but note that the edges are sloped (on old and new) to identify which way around it goes - look at the data sheet on the RS Components part page for diagram.
Bend legs to match the old device, get out your smallest soldering iron (the device is 3mm x 4mm) and take out the old and fit the new.
Get someone with good eye sight to do the job or to check that the solder is not bridging the small pads.
Refit the collar - you will need to compress the brushes back into place to get it to fit over the commutator and the magnetic ring.
Reassemble motor and test - job done.
The black ring next to the commutator has magnets in it. When it rotates it influences the hall effect sensor and lets the CCU know that the motor is moving. When the window hits top or bottom the CCU sees that the motor has stopped, this calibrates the CCU as to where the rear window is. When you open the rear door the CCU sends a signal to the window motor to rotate a number of times (haven't worked out how many yet !) and the window drops to clear the rear door seal.
Hope this saves someone the £30 odd quid for a new mechanism and / or motor.