I assume the key-operated lock is not working.

If so, likely a clip holding link rods + levers together inside tailgate has come loose. They are snap-clips; slamming the door very hard can make them pop off.

The cure for this requires removing the inside panel (remove inner latch plastic cover etc. first), then using patience and very flexible wrists reach around in there, find what's come loose and put it back together. Any loose clips usually fall to the bottom of the door; that's where you'll find them.

Be careful of the "child safety lock" lever, located on outside bottom of tailgate. Turning this disables the inner latch opener. If you use it, and the outside opener/lock has a mechanical failure, you could find yourself locked out more thoroughly than you want.

If only the power locks aren't working, there's probably a broken wire in the wires that pass through the hinges. On my '89 the right side hinge had the wires for the power lock.
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Sven: '89 245, IPD sways, electric rad. fan conversion, 28+ mpg - auto tranny. 850 mi/week commute.