I installed my empeg in the trunk of the S4 where the nav system is supposed to go, right below the cd changer. I'd like to install it in the dash somewhere, but I want to keep the factory head unit, and I can't find room for it with out cutting. Someday, someday :)

I tapped into the cd changer wiring, using a relay operated A/B switch, so the empeg is accessed by selecting the CDC button, if the empeg is on, the cd changer output is routed to the AUX input on the empeg and the empeg output is routed to the cd changer input on the head unit, if the empeg is off the relay switchs the cd changer output back to the head unit.

http://tychl.txf.com/S4-EMPEG.jpg
schematic of the A/B switch and a bad drawing to the cd changer pinout on the vehicle side of the harness.

I might post pictures in a few days, after I get the MK2 in place.