There may be some year to year difference on this.. this is what I need to do to my car, I don't see why this won't work for you.
1. Remove wood trim panel [may be black or gray for you]
2. Remove the screws holding the dimmer switch in place
3. That'* it
Check all the connections, making sure they are all secure. If there is no corrosion on those connections, chances are there is wiring problem somewhere. Wiring troubleshooting is very, very hard on any car, and especially difficult if you cannot rip apart stuff to get to all the wires.
Unless you really know what you are doing, I would suggest against removing the dash. It is NOT easy. You will have to work underneath the dash, with the black [mine are black, at least] that give you semi-access to the wiring. It should be enough to see if any wires are broken somewhere down the line.
Otherwise it could be the switches themselves. You may want to go to a junkyard, and pick up the dimmer switches, see if that works. Or if someone knows how to test these switches, they could help with that. I know on mine, there are four pins for three connectors, and although it would probably take me a few minutes to figure it out, it would be much easier for you, if someone already knew.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.