retrospex

Hey guys,
My friend is in the middle of a custom stereo installation for a G37 coupe and we need to find out where the speakers receive input navigation and parking sensors so you can hear the navigation prompts and beeps when you are parking. More specifically, the center speaker is what we're trying to make work properly. Everything was disconnected and we're working to get things back in order.
Our guess is that the center speaker (which has a lot to do with the navigation/parking) gets it's input from the rear speakers, which is linked to the parking beeps and navi prompts.
Any help and knowledgeable insight would be much appreciated. Thanks in advance!