Hard to know exactly. I work on these and I am not sure if I have come across this problem. The rear wiper motor has a tendancy to fail, so it could have seized. Possibly the latch has failed or you may have a broken wire in the passthrough boot from the liftgate to the body. It may not relate to the latch problem but check the 3 amp fuse for the rear wiper in the fuse block on the right side of the centre console at the front.