I have a theory… nothing more than a theory, mind you, but a theory that we can unmount the CD changer assembly from the upper assembly and leave that in place to retain the HUD/onstar/etc functionality. I'm pretty close to certain that the only info coming out of the 32pin connector that goes to the class2 system is the radio and nav-related HUD data.
Check out those wiring diagrams I posted: All the audio goes into the CD changer before it goes to the amp, and the steering wheel controls are connected to it as well. The only signals that go via the 32pin besides power and audio preamp output are the VSS, the triggers for the radio antenna, aux trigger, nav voice guidance and the microphone.
We can ignore the nav voice guidance wiring since that's irrelevant for the appradio, the VSS can be used by the appradio, and I'm willing to bet we could even use the built-in microphone if we wanted to. The only unknowns that leaves are the radio antenna (the aux trigger is what informs the head unit that there's an XM receiver hooked up, I'm pretty sure).
The caveat, however, is that I don't want to tear apart my own head unit apart to check this out, and I haven't even taken it out of the dash to see how easily the changer separates from the display/nav/radio.
