I have an '07 A4 w/ the Symphony system and I'm attempting to install an after market headunit.

The problem that I'm running into right now is that I can't find an Auxiliary wire. I know that there's got to be a keyed wire under the steering column, or possibly an adapter that could "create" an Aux wire using the cars electronics system.

Could any of you give me an idea of which direction to go?
If you suggest using the steering column, what wire would it be?

Thank you.

EDIT: Here's a picture of the cars wiring, if that helps.

 

My understanding is that Audi uses a digital bus and sends signals to control the electronics - no more old-school voltage level controls. Somewhere on here (or maybe it was avic411.com) there are posts about an aftermarket control box you can use to intercept the signals and give you auxiliary (amongst other things). Kinda pricey though...