I have heard that the SC unit needs to send/recieve information with the CD Changer, which is why you can get an "EEEE" message even when you have power. I once read on this board that you need more than just the power/audio connections to the DIN jack, there is also data which needs to be sent for the Changer to work. Kind of makes sense when you figure that the head unit will control the tracks, discs, etc. That's also how you get the time and track display on the head unit when using the changer.