My rear wiper stopped working recently. The washer still worked. I found a blown 15a (blue) fuse in the rear luggage area fuse panel. I replaced the fuse and the wiper worked again. There are spare fuses and a fuse extractor tool behind the panel accessible on the side of the dash with the drivers door open.
About a week later I went to the airport to collect some friends. I had opened the tailgate at home to empty the boot ready for loading with suitcases. When I got to the airport I could not open the tailgate. Further investigation at home later revealed the wiper had also blown its fuse again. It was the wiring loom at the tailgate hinge that had worn out and wires were going short circuit.
If the wiring loom is damaged in addition to the rear wiper and tailgate locking you could also lose the top LED brake light, or number plate lights, or rear screen de-mister. You could also get a tailgate open warning on the dash display even when the tailgate is closed.
See
https://www.volvoforums.org.uk/showthread.php?t=285157 for more details and photos.