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Failure of the lock/unlock relays is rare. Failure of the switches in the driver's door, or a break in the wiring where it passes through the hinge area is not uncommon.
Assuming your 1990 is the same as the earlier years.....
Before pulling the door panel try an easy check: remove the black felt pad above the driver's legs, and use a flashlight to locate a trio of wires (Red, Black, Green) going left-right and joined in a white nylon connector just to the left of the steering column. Unplug the connector. The wires going to your left go into the door and to the switches on key cylinder and pushbutton. The wires going to the right and disappearing up into the dash go to the relays.
Referring now to the connector to the right, and using a paperclip, or anything metal, short the Black wire in the connector to the Green. The doors should unlock. Short the Black to the Red - doors should lock. Only tiny amounts of current are involved in the wires you are shorting, so don't worry about sparks or heat.
If everything works, the problem is inside the door, so you'll have to pull the panel and start looking around. Plenty of posts in the archives about this.
If the unlock function does not work when shorting Green to Black, there is a problem with that relay, or connections. You have to remove - or at least pull back - the centre console to get good access to the relays and their harnesses. They are small metal cubes, clipped to metal framing in the dash, just below the centre air vents. They are SPDT relays with a centre terminal labelled "87a", Widely available at auto parts places for about $7 or electric supply houses (Jameco.com) for about $2.50.
(Right beside the central locking relays is the power window relay if you have PW's - but it's shinier, and has sharp, square edges.)
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)
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