I don't understand what a NAVI device is but I think you want to mute your car stereo when the NAVI is sending you a message.
You might use a relay to disconnect the speakers from the stereo player during this period. This won't require tearing apart the stereo.
You will also need an amplifier & speaker for the NAVI and a control circuit to drive the relay, probably audio derived.
Any of this make sense - work for you?
I've spent a few days researching.
A navi is short for navigation device. Kind of like a TomTom-brand product, but in the form of a PDA and phone. It has its own output, "headphones," that I was planning on using. I also actually wanted to have this audio channel play over the two front speakers instead of the head unit when it had something to say. The music can keep playing in the back, I just need the directions up front.
I'm not sure what a control circuit is. I'm going to put the device, if possible, between the head unit and amp because the audio coming from a head unit and from an MP3 player, for example, is the same,
right? (This assumption, which I'm questioning now, might require a redesign.)
I looked up what a relay is, and it seems good for this project. Would an electromechanical relay or a solid state relay be better suited?
How about a PNP transistor? Since I want to disconnect the HU from the output upon detection of a signal, wouldn't a PNP transistor do that fine? Also, that voltage controller could be a potentiometer. If the potentiometer and normal channel are both connected, wouldn't the flow go around the potentiometer, eliminating the need to add an NPN transistor?