I had a rear doors not being able to be unlocked problem with my '81. The reason was seized lock motors.
Here's how I solved the problem: I pulled up the lock knob as far as I could and held it there while I tried the door handle, luckily the door opened. Then I removed the door panel and popped the linkage off the motor. The doors can now be unlocked. My central locking doesn't work so I just left the motors in the doors.
--Bill
(I had previously posted this as a reply to different topic. Thought it'd be wise to make a new topic so someone else with this problem would be able to find the solution easier.)