Know of a way to mute a car deck with 3.5mm or USB on HTC?

[Mysticle31]

I have a Touch Pro. I have been searching for a car stereo that will always pass-through aux or bluetooth so that I can listen to a source (CD, Radio, MP3..whatever) and still have GPS directions heard over speakers.

Since I can't find a deck that will do this and I can't find a way to hack it with my current skillset, I'm investigating splitting the input on my amp so that there are two lines in, one for the phone and one for the deck. Now I just need a way to mute the deck when the phone wants to output sound. Any ideas how I can do that? Modules, Auto sence input adapters, sensors, switches, relays? I don't have a mute out as you may find on some dedicated GPS.