audio activated switch

Peripheral and PAC both make a unit that does this. Dei does as well...part # 55000 "remote power adapter".

However technically it doesn't use the "sound" of the stereo to turn on the amp..well if you were using amps for all the speakers you would not have any sound to turn anything on. What it does is sense the audio signal from a speaker output of the head unit and send out 12v+ to turn on an amp. Very handy piece.

At the PAC store...kidding. Any local mom and pop car stereo shop should stock these..if not they should be able to order one for you. Some of the larger retailers like circuit city and bestbuy should stock these as well.

If you are not able to find one locally just do what everyone else does...ebay it.

So I just see the PAC-Man Guy. LOL

I have not used a "light to the beat" thingy in a long time..but they are still out there. Street glow and lite glow both make a unit that does this..called a "music interface"

http://www.optxbysg.com/opmi.shtml

However the street glow one has to use neon lights, I am not sure about other brands or whats still out there.