I had my TJ for three year now, and I've had one persistent problem being that the back-lighting in my instrument cluster and A/C controls have not worked for as long as I have had it. I have checked and rechecked every fuse, connector, wire, switch, light bulb, relay and ground in the dash and under the hood. From there I replaced my multifunction/ dimmer switch, but that also didn't work. When I first bought my Jeep the previous owner had hastily installed a JVC head unit, which I rewired, however it still has three unused wires orange (dimmer), light blue (power ant.?), and brown (not matched to a wire on the factory radio plug) I don't thing these wires would cause my problem, but I could be wrong. One other thing I noticed is my TJ doesn't have a security bracket or large radio ground wire behind the radio, possibly due to it being a 2003, but I'm not sure about that either. Any help or ideas would greatly be appreciated. Other wise I would have to believe the previous owners alien encounter/ abduction story.
I don't think the head unit is causing your problems, wiring sounds pretty normal. On the other hand I have had problems with those little light bulbs that go behind the instruments needing the mating surface of the board cleaned very carefully to make good contact. Just takes a tiny bit of corrosion to stop them from working. Although that was always one bulb here and one bulb there, as a matter of fact the one behind my fuel gauge is doing it right now but I have just been hitting the top of the cluster to get it to come on for the last year or so. Lol. The bracket behind the radio wa smost likely removed during insttallation of the new head unit. Maybe he was abducted....
Unfortunately, the dash/cluster illumination is driven by a driver circuit inside the cluster. The driver circuit is known to fail in some TJs, and the circuit is unrepairable as far as I am aware.
If you don't mind the dash/cluster illumination being lit at full brightness all the time, there is a work around that can be done. The black/yellow wire that supplies the front marker lights and tail lights can be tapped to the orange wire that supplies the illumination for the dash/cluster - one section of which you have found behind the radio. It's obviously not an ideal solution but it has been done before, works, and I've heard of no problems as a result of this work around.
I'll check connectors in the fsm later. . The black/yellow wire from the multifunction switch supplies the front and rear marker lamps so needs to be tapped rather than cut. The orange dash illumination wire could just be connected near the radio if it's not in use there. That will then supply all the dash and cluster lights.