Probably a fair assumption, but it is a parallel system. It's probably a pinhole leak at the actuator or close by downstream. It subverts the signal to that actuator enough for it not to work, but it's not big enough to affect the entire system.

I think the easiest approach is to pull the door panel on the affected door and check for vacuum and pressure feeding that actuator as the doors are locked and unlocked. If there is not vacuum and pressure, trace the line to the branch. If there is vacuum and pressure, replace the actuator or do a search on George Murphy for replacement diaphragms.

Sixto
91 300SE
87 300SDL
83 300SD