Hey, I just bought my 91 model. It had the same problem. I checked fuses and such. Still nothing.
I found 2 wires disconnected dangling on top of the tranny behind the engine. Was visible from the engine compartment.
I thought maybe they plugged in to each other as one was a male and the other a female connector. I was looking in my book, at the diagram.
I saw next to my reverse safety switch was the 5th gear switch. (Still have no idea what that's for-why a 5th gear switch)? I was feeling around, along the wires and they went only to the 2 switches or sensors. (whichever)
I looked around wondering what they plugged into, I found a male and female connector on the harness above their location on the firewall. I made sure they were clean and plugged them in. VUALAH! REVERSE LIGHTS WORK!
Try taking the connections apart and clean them up. Make sure there is no corrosion in the connectors, before you plug them back in. Also might want to inspect the wiring for damage and put an OHM meter to them also. Check your resistance as well.
The reverse safety and 5th gear switches are on top of the tranny towards the front end. If your arms are long enough to reach, you should be able to feel them. You might have to lower the tranny down to get them out, if your switch is the problem.
Let me know how things go.