Sounds like they went in to the dimmer line for your head unit, but connected something wrong. Anyone who does this for a living should not have any problems hooking up the dimmer line. I have seen amatures incorrectly wire the dimmer line on the after market h.u. to the 12v acc, so when ever the radio is on, the display is dim. But unless they shorted something (sounds like that's what happened), I don't know how they burned your entire dash illumination.