Rear door will not lock

[planner111]

I have discovered a problem with a rear door lock on my 82 240d. Not sure if this is a vacume problem or a mechanical problem. When I use the key on the drivers door all the locks go down as they should. However the rear door on the passenger side is not locked. The lock handle is in the down position but it is not locked.

When using the lock manualy it freely goes up and down but will not actually lock the door. All other doors locks work as they should. If I take the panel off where should I begin to look?

Thanks.