The sensors will be a main dealer only part, there are three part numbers depending on your chassis number
4B0919275A, 7M3919275 or 7M3919275A, they can be replaced without removing the bumper
Note:
It is not necessary to remove the rear bumper.
First switch off ignition and all electrical consumers and remove ignition key.
Press back both sender securing tabs.
Press out sender inwards from outside of bumper.
Remove connector.
Have you checked the control unit connectors in the boot behind the right side trim?
nigel
November 2011