Sorry, I have come quite late to the party here; acknowledging the reference to some of my work on small motors in post 2. Thank you.
That was the headlight wiper motors, which are NLA. There may be some transferable information about the sort of problems and how the motors park. But they are quite a bit different.
https://www.volvoforums.org.uk/showthread.php?t=330379
I've never opened a rear wiper motor as none of mine have failed, yet. The Volvo part number is 1358011. OEM, pattern parts and refurbished ones still appear to be available. Skandix have a good range of options.
I think, from other posts that I have seen, problems are often due to the linkage coming loose or needing fresh lubrication. So it would be well worth checking that carefully before doing anything too drastic to (or replacing) the motor.
If I were going to try dismantling one of those motors I'd be looking at drilling or grinding the rivets on the gearbox in order to get into it and then probably tapping the holes in the body to take a machine screw to reassemble. Or maybe just a suitable nut/bolt.
That said, with replacements available for under 100 Euros I'd probably just buy a new one and put the original on my ever-growing pile of things to look at one day.
Good luck to the OP fixing it.