+1 on AndrewWynn's comments above
I repaired my driver's door lock actuator - pics and info in that here:
https://xoutpost.com/bmw-sav-forums/...or-repair.html
(when I did it, I incorrectly assumed [not that it mattered] the two motors were for lock / unlock, but I believe Andrew)
A major point I will add to the comments so far is that in a separate issue I fixed with the central locking system (turned out to be an electrical problem in the driver's DLA), the rear door lock actuators were functional but weak. For example, unlocking half way rather than fully. I was convinced they would be needing replacement due to a similar problem as I had found on my front driver's door, but as soon as I fixed that other problem, they were working perfectly again.
So the take-away is that one problem in the central locking system can create weird symptoms in a completely different location. The fact that you have one door malfunctioning rather than both rears does support the failed motor theory.
The motors run on 12V. So I would get access to them by removing the inner door panel, and apply 12V, positive and then negative, and see if the actuator is solid or flaky or dead. I don't know the pins to apply voltage at.