Help. Our central locking (e34) is beginning to act up (getting weak)

I am getting ready to rehab our 1993 525 (M50 motor) and need some assistance so I can speed up the process.
Last week the right rear electric door lock stopped working with the
central locking function (you can still unlock it by hand). The other three door locks still lock/unlock from the driver's door, but they seem to have gotten MUCH quieter (weaker??). Has anybody ever experienced this?? Any suggestions?? I don't want to start replacing the actuator(s) unless it/they is/are at fault since they can be a bit pricey. How can I diagnose?
I am posting other questions since I have one week to replace the hood, grille, front left fender, windshield washer reservoir, valve cover gasket, ashtray frame, rear window motors and regulators, driver's seat belt receiver, rear right door seal, and wheels.
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