I replaced my stock Delta CC (Blauplunkt) head unit in my 96 A6 with an aftermarket head unit. The front amplified channels of the new unit was a direct connect to the front door speakers. Which I replaced with an aftermarket 8 ohm 2 way system. (the stock speakers are some 2 ohm odd ball type that is not compatable with the new head unit that wants an 8 ohm load.) I then had to connect the rear pre-amp outputs from the new unit and connect them to the wires for the rear door pre-amp inputs (located in the connector of the old head unit). You then need to find the wire that switches on the power to the 2 rear door amps.(also found in the old connector) This is not the wire that powers the door amps but just the switching powerthat turns them on. I even removed the rear door panel to get access to one of the amps and its connector, just so I could verify the wire color coding and measured with a volt meter to find the switched 12volts. I did this with the origional head unit, because I knew that part was right. The switched power then is connected to the new head units switching power (sometimes called the "antenna power"). You also may need to connect this same switching power to your powered antenna amp if you have one, mine did. Hope this helps. It may take a little extra effort to find all the correct wires, but its necessary to make it all work. btw... the new system blows away the tired old origional.