installing two head units

Has anybody installed a second head unit piggybacked onto the existing one? I have many customers in particular UK spec Toyotas where a double DIN space is available and they ask if the original Toyota radio/cassette unit can be left working along with a new CD player underneath. I was thinking maybe the amp remote trigger could be used to switch a relay using 87a to power off the Toyota unit when the CD player is switched on. My concern is how to wire the speakers to both units. Could it be done using diodes?

Any help greatly appreciated.

BigAl

Halfords UK

If you wanna go overboard and scare the living sh*t outta your customers you could use relays to seperate each speaker wire with a relay so it will only actually connect one set of wires at a time. When you throw a switch it would throw all the relays. Of course the look on your customers face would be pricless when he threw the switch the first time and like 10 relays started clicking around his dash. I dont see why the 3 way toggle switch wouldnt work except I dont know what kinda reprocusions could come from driving signal back through the deck that is not on. I would assume that since its off it wouldnt hurt it but like I said, I never tried.