The wiring diagram for my factory radio is here. You can tell which specific radio you have by looking in the glove box and seeing a bunch of 3-letter RPO codes (GM) and matching, or by follow the picture here. In my case I have a US8 radio (from the sticker in glove-box):
Which in turn has these features (story checks out).
Audio system, AM/FM stereo with MP3 compatible CD player, seek-and-scan, digital clock, auto-tone control, Radio Data System (RDS), speed-compensated volume and TheftLock
So the X1 and X2 connectors are present (no X3 or X4), and pinout as per p C-64 and C-66:
So once i sort the OTG charging I will obtain some connectors. Note I also have UE1 and UQ3 which are mentioned on the connectors). The UE1 comes from the OnStar bluetooth kit (I think) and I plan to use that as the bluetooth headset adapter input (since there is no current support for the Headset Profile 'Server' in Android, see http://nohands.sourceforge.net/).