I have a 1992 Volvo 740 turbo, and recently after noticing somebody has “broken” into my car, I checked the locks and see that the rear driver side lock doesn't lock when i use my key. All other locks go down, the problem lock slightly moves, and i can hear the mechanism on the door, but it doesn't lock fully. Is this possibly a wiring issue? Or perhaps an actuator problem? I am no mechanic but I can remove the door paneling. It is the weekend and I would like to try and resolve this before there is another night prowler opening my car doors.

 

sounds mechanical to me.