Cadillac Repair: Illumination Problem, illumination problem, aftermarket stereo

Question
I recently acquired a 1989 Eldorado and non of the illumination is working.  Checked all fuses inside and out, everything is sound.  The previous owner installed an aftermarket stereo and i have yet to remove it to look for possible mistakes in wiring.  Could this be the dimmer switch by chance?  The illumination for the ac/info center aren't on either, but all controls are working.  This is my first Cadillac and I'm lost.  Thanks.

Answer
The most common fault is that he probed the wires for the new radio with a test light and there are 5 volt, battery voltage and 16 volt wires back there that went into the old radio for illumination and he most likely blew the instrument cluster, If DIGITAL and or the computer.