Check out this diagram of the BMWPIO adapter cable.....
Couldn't you simply splice into the pins 1, 2, 3, & 4 and use a 4 pole switch or relay to go between the CD changer signal and the iPod / mp3 player signal.
http://www.cambridgebrands.com/1995-...o-adapters.php
Of course the tuner selection would have to be set to 'CD' and the iPod / mp3 player would have to be controlled right at the device and not from the deck (like with the CD). A nice toggle switch or button used with a 4 pole, double throw (12VDC) relay could be slick.
I have a RCA Lyra 1080 with a 1GB SD chip (nearly 600 songs) and I would love to find a cleaner way to use it with my car than the tape adapter.
Since this radio system used M-bus, it is a lot harder to use the deck for control without making an inteface box ($$).