i would say there's a short in the radio wiring, or maybe power backfeeding from the radio to the interior lights. on the factory head unit, there is a wire to dim the radio lights when the headlights are on. also, i'm reasonable sure there is a courtesy/ashtray light that is in the same wiring harness... best as i can figure, a couple of these wires got together some where, and is causing your issue. pull the radio, start tracing wires.