OK so the next thing I tried was to remove the radio, and just using a factory style plug (not the adapter harness), run 12v power, ground, and GMLAN (3 wires) to the diagnostic port under the driver side dash. The radio comes on and works properly in ACC, RUN, etc like it should. So there's apparently an issue with the adapter harness. But what I can't figure out is, what could possibly be mis-wired to cause it to behave like that? I have to pull up wiring diagrams so I can see what's what.
Here's the other thing - I finally got a hold of the guy who configured the radio and sold me the harness. He specializes in GM radios. He mentions "Well, maybe run a wire to the accessory power etc etc" and I'm just.. sigh. You SPECIALIZE in OEM GM radios! There's no accessory wire! Why does nobody know what they are talking about? If you have a GM car since 2000, or a GM truck since 2003, there's NO accessory wire. There's 12v constant, ground, and a network wire (class 2 or GMLAN). The radio is always receiving power. Always. It just listens for a signal over the network to actually turn on when the BCM tells it to. There's no way to turn a factory GM radio on any other way besides with the command sent from the BCM. Period. It's just frustrating when you want someone to say "Oh, right, I know exactly what you're talking about since I do this for a living" and instead you get an uninformed answer.
