Intstrument Pane / Guage Lights do not work???

Most likely the dash dimmer is crapping out. You might be able to mess around with it and get the gauge lights working but at some point you are probably going to have to replace it. It is a pretty common occurance with the fox Mustangs. My 88's is really touchy. If you try to adjust it the lights go out and the dimmer isn't cheap either.

I can't remember which way it turns but when you turn the dome light on you can here it click and the dome light comes on. When mine went out the dome light function still works since that is the switch. There are actually two different parts to the dimmer switch. The dimming function for the instrument cluster and then the dome light.

What you might want to do is pull the dimmer switch, unplug it, turn the headlights on and then with a wiring diagram jumper from the power coming from the headlight switch out to the instrument cluster lights. That would check the wiring and make sure that the switch is the problem.