Hello Palmer,
When I got my 92 244 last year, the backup and brake lights were a mess. I am not sure but I think the 89 is like the 92. I had a bad neutral safety switch, a burned out ckt board, some bad socket grounds not to mention a white aftermarket brake light socket that should have been black. You can do any of the following in any order.

Remove the four screws that fasten lenses on each side and inspect the flex circuit board for burned out circuits and the ground integrity. These are about 20 bucks each if bad.

The reverse lights are not part of the bulb out relay circuit so you can rule that item out. There is a single red/black wire that feeds the reverse lights from the neutral safety switch (automatic). Check for 12V when in reverse. I tracked my issue to the neutral safety switch.

For the tail lights, if the front running lights and your license plates are working that means the "main" relay is working. I’d have to then focus on the bulb out relay. Check for voltage at the brown/white wire for the right side and the red on the left. These both go to the press on connector on the left side.

To test the sockets, I connected two wires to the plus and minus battery terminals and tested each socket with the light in the socket. I found a couple with weak grounds and one with no ground contact. It is probably easier to do this an ohm meter but I was sorting out lamp types by brightness at the same time. If there a poor contact you can bend it back in place.

Tom