The only slight downpoint is that you won't have music in the car without the Empeg.

This is the same prob I have with my installation; with only one empeg available between two cars, my wife still wants to be able to listen to the original head unit (radio/CD) when the empeg is not installed (presumably in my car).

I'm working on a relay-switched switchover network that will re-route (by default) the audio path from the outputs of the head unit to the amp when the empeg is not present; otherwise, with the empeg inserted, the audio path is switched to the inputs of the empeg, and there out to the amp.

Any recommended audio path switching mechanisms, Dominic?