I would imagine that the adapter is actually processing/sending messages so it may not function properly without the ibus connection.
When the CD 01- comes up on the cluster that is actually the radio responding to the steering wheel button press. It sends a broadcast message to all modules on the ibus based on the selected source - ie. when CD is selected, you get the CD 01- and station frequency for the FM radio. It's possible that it doesn't do anything when the tape source is selected, but I don't own any tapes so I can't test it. Unfortunately, there really isn't anything that can be done about it. It works in reverse for me, the CD 01- comes up and then the trackname message blots it out.
Attached is the latest version - nothing new in it, I've just been cleaning up my code a little. (Can you tell my wife has been out of town?!!!)