The same fuse (on the '84, that is) operates the radio and interior lights. If it's not the fuse, the lock/unlock switches are prone to fail. It could be that both failed and the owner just let them go. Next: the switches do not operate the door locks. They operate a relay that operates the locks. The relay is behind the panel just above the radio, and easily accessible. A failed relay or failed (plug in) connection at the relay could be the problem.
I would expose the relay and jumper 12v to the activator wire (black, on an '84). If the doors lock, the door switches may well be the problem. If they don't lock, I would pull the relay from it's connector and visually inspect the connection. It should be shiny brass colored. (Maybe copper. Okay, I'm color blind and can't differentiate.) If it is, I would suspect the relay itself, but before I go buy one, I would test it on the workbench.
Good luck.