Why would CAN be required? Many reasons, a few being:
Navigation, speed correlation, settings customization, satellite radio control, onstar interoperability, split sources between seats, play movies to front verifying not in gear and speed zero, play movies to rear, data bus to CD changer, data bus to map
Disk drive, theft prevention, communicate to body / engine ecu for power requirements, amplifier setup & config for occupancy, dim lighting, environmental controls, and so on.
Use the upfitter manuals for year and model type of donor, heck even build date of radio plus a year or two. You will have your pinout. If have RPO codes (in glove box), then search for those plus upfitter manual.
You could also maybe call GM and ask nicely. Stealerships may help as well.
CAN isn't all bad, though does get highly annoying.
~SD
