I managed to do exactly what you want to do with a couple of relays in a little black box and a little basic soldering. It worked great in my Range Rover and cost about a tenner.

My wife was still able to use the radio/cassette/CDchanger when I had the Empeg in my car, which was the main point of the exercise.. I also worked out a circuit to feed the HU through the aux with no switches, but decided to sell the car before I got round to fitting it.

You'll find more on a thread very near to this one. I'm not very high tec., if you want the circuit, send me a personal message with a fax number!