MP3 Integration question... looking for a 12V trigger module

[Q451990]

So I'd like to be able to hook up my phone to the stock head unit using the headphone jack. The problem is switching the radio over to the CD input. The head unit needs to be triggered by 12V to switch to the CD changer input. From there it's just feeding it left and right line level audio.

So my question is this... does anyone know of a module that would sense the audio signal from a phone or MP3 player, and then output 12V? I know I could rig up a toggle switch, but that would be too easy.

Heath