Heidelberg VII and VII in early B3

Has anyone ever put a later model Heidelberg radio in a car a couple of years older? I am trying to get radios from 1992 and 1993 Passats to work in a 1990 car. The Bentley shows some additional/different circuitry tied into the ignition and seatbelt warning processor, and another connected to the speed sensor/cruise in the later cars. The 1990 doesn't have these wires at all, but all else seems the same. When I plug the connectors in, the unit won't even turn on. The little anti-theft light blinks, and power is present at the connector and fuse in the back of the unit. The 1993 radio (not labelled Heidelberg...has CD change function) makes a kind of warming up noise when the plug is inserted, but the same problem...the switch does nothing. I can't even get to the SAFe code!
Any clues on the problem or what to test? Or; do I need some additional connections?
Thanks for any insight.
RTM