Hmm good luck with that.. I was ecstatic enough when I discovered this:
I used my Tilt wih the TP headphone/charger adapter and hooked it up to the AUX IN on my head unit, and when a call comes in it plays the ringtone and acts as a speakerphone through the speakers and automatically pauses the song on WM!
Of course, it's lame that if you have the phone sitting in a cupholder, no one can hear you talking!
lol
Anyway, is your GPS a separate unit or are you using the Touch Pro unit by itself for gps?
Do you want the source to mute whenever the gps talks?
It'd probably be helpful to explain your layout or maybe I'm really tired and am trying to sound smart lol. And also check out car audio forums.