96' Back light for insturment cluster won't work

This is actually for my friends car. He bought a deck and amp system for his car, and I am a somewhat profesional installer so I wired it in for him but after I was done with it the streo worked great but the back lights for the spedometer etc. no longer came on when the headlights were switched on. My first thought was fuse and after checking all of them and finding one burnt out in the fog light slot (the vehicle wasn't outfitted with them ) i thoguth they might have a shared system so replaced it and it still doens't work. So i'm stumped and so is everyone i've talked to so far. So if anybody has some suggestions or even some commicerations they would be greatly apreciated I need to salvage whats left of my reputaion.

check the dimmer switch, may be messed up. did you use the illumination wire from the harness on the stereo somewhere?

i just had the same problem, my dimmer switch was bad, but when you go to order a new one, its not called a dimmer switch, it has some wierd name, if you tell them dimmer switch, they give you somethin else

Oh my god I just had this same problem happen while installing a radio into my girlfriends car right down to the same fuses popping. Anything ever pan out for you on this issue? Was it the dimmer switch? If so how much are we talking here