If your rear windscreen wiper will only move at a push, it may well be a corroded spindle. Option one is to buy a new unit. Option two is to dismantle it for some TLC and to free the mechanism.
- With the boot shut, prise the plastic bit off the wiper arm to expose the nut, then undo the nut
- With two hands grab the wiper arm and wiggle/ pull the arm off the spline. Once off put the nut back on.
- Unscrew the phillips screw from the tailgate hand grip and pull all of the black plastic loose and away from inside the tailgate
- Take the connector off and unbolt the wiper unit.
- Take the wiper unit to a workbench to undo all the torx screws and open it up for a clean. Take note how it goes back together! (photo, marker pen)
- with the nut on to protect the threat, tap the shaft out of the housing. This may well be heavily corroded, hence the wiper motor cannot turn it any longer
- sand the corrosion down and grease the shaft with lithium grease, as for the wiper mechanism
- reassemble and remount in reverse order.