Thanks to this board, I've acquired a Foxwell NT520 Pro. It will let me command sleep mode on demand. So closing the door latch and commands, I can do further testing. If wiring diagrams show it's on the same module, I'll definitely unplug it and test sleep mode. By information I've found, it seems I'm down to the fixing the consumer cut off relay inside the GM3. I've replaced a few parts and pulled a few fuses for testing sleep mode.
I learned about the rear final stage resistor from a YouTube video.
https://www.youtube.com/watch?v=UK_aO_TV-Rk
The video shows all the dash modules and locations.
The passenger seat may have to come out to reach it.