Hmmm
http://www.alpine-usa.com/US-en/prod...odel=VPA-B211A
it's a little expensive to do what you want.
Blaunpukt HU's offer 2 aux ins on some models
http://www.blaupunkt.com/us/7647643310_main.asp
But I would just build some relays tied to a switch and call it a day. Would cost <$15-20 in parts.