[s-cars] CD changer interface questions

[Kevin Day]

Does anyone here know any details about the CD changer interface?
Specifically,

- How does the head unit know there is a CD changer present?
- How does the CD changer know that the head unit is in CD mode?

The reason I'm wondering is twofold. Firstly, I have an mp3 player
connected to a relay (via its amp remote output) which selects between
CD changer audio and mp3 player audio. This works fine, but when I'm
listening to mp3s (nearly all of the time) the CD changer is playing,
switching CDs, etc. It would be nice to avoid that wear and tear. The
second reason is that my power antenna keeps jamming, and I'd like to
gate the power antenna signal with an inverted 'CD mode selected'
signal. I.e. don't raise the antenna when the head unit is in CD mode.

This is for the 10 disc Gamma system.

Thanks.

-Kevin
'94 S4