That's interesting for anyone wanting just to connect a mp3 player to the head unit letting the control cables run to the changer as I did...
BTW, FYI, here are the main components used in the Connects2 interface (I did some reverse engineering on the hardware).
TL064 (stereo AF amp), linked to a 5331AKS (stereo A/D converter with serial output), linked to a CS8405A (SPDIF encoder), linked to the HU's SPDIF input.
UART control port:
ST72C314J4 (some flash microcontroller), linked to a BA8270F ("Bus Interface for Car Audio") which connects to the HU's UART lines.
Datasheets of these chips can be found online. Let me know if you have trouble finding them. Given your theory about the protocol, the BA8270F might be particularly interesting, as the datasheet states that it's an interface *master* :)