I've spent a few hours inside my car radio and I've found that when I short a hidden pin called AUX-ON to VCC, I can pipe music right into the it and it's treated like a regular AUX input. Great success.
Now, I have a switched jack that I'd like to use to short VCC to AUX-ON whenever I plug in a cable. The jack I'm using is Radio Shack 274-0246, the diagram is below. I can't figure out any way to do this without having the potential of grounding the audio to the vehicle ground (which could, I assume, distort quality) or without having the potential of sending 12v back through the audio cable into the phone. I know there MUST be a way to do this with a semiconductor or two, but I don't even know where to start. All I know is I need to pass current from pin 2 to 3 (or 4 to 5) and when that current stops (the jack mechanically separates the contacts when plugged in) then VCC can go to AUX.
EDIT: I forgot to mention, the unit is expecting balanced inputs, however.. I don't think I've ever seen an ipod or phone with balanced outputs. Should that be taken into consideration with this circuitry?