getting around factory amp issue
here's my situation, my particular car needs the factory amp because the navigation phone and all the other bells and whistles run through it. I'm upgrading my system and really don't want the factory amp but I want my music to attenuate the same way when the phone call comes in or the voice navigation comes on. So what I'm thinking about doing is taking the speaker lines that come out of the head unit and go into the factory amp and route that to a junction box (I'll get back to this) then out of that directly to my aftermarket amp. Then taking the line out of the factory amp that would go to the speaker, and send that to the junction box. In this box I'm imagining something that will see (audio) voltage from the factory amp carrying the phone signal, navigation, and any other voice directive coming from the car. when it sees that voltage it attenuates the main audio signal coming from the headunit and patches the phone or whatever else in.
Here's the question, what do I need besides a relay to build something like this? Where to begin? Will this work?