I put this in here because it's more about lighting than audio.

I don't know whether it's legal, but has anyone ever gotten a sound sensitive module for their car's lights? I got one for the lights in my computer case for $10, basically it flashes the CCFL tubes to the music that's playing. I think it'd be awesome to have flashing neons inside your car to the music, probably far too distracting though.

Cops are on to it these days. Fine for at shows and no good when driving. Jaycar sell sound modulators