Found a page that seems to list the pinning of the CD-changer plug.

Alfa / Alpha Romeo (pm)
Alfa 937 High CD (Blaupunkt)
Die Pin-Belegung des CD-Wechslereingang am Radio
13 - Data in
14 - Data out
15 - Spannungsversorgung, UB+
16 - Masse
17 - Schaltspannung
18 - Audio Masse
19 - Audiosignal rechts
20 - Audiosignal Links

No idea if the radio requires a changer (or an adaptor faking it) to be present or not in order to accept input on pins 18-20... Can you select AUX In or CD-changer today, without a changer being present?

/Michael