Car has had intermittent variety of problems. To keep this easy to follow I will not list everything unless asked.
Car now has persistent symptom. Great! Should be easy to find right? Not really.
Main issue seems to boil down to 12V output of BCM on Connectors X3 pin 12. And X1 pin 17. Both are tested bad at less than 1V closer to 0V.
Have double checked the powers and grounds to the BCM. It communicates. Tried to replace and program BCM. Will not complete the process.
Now suspect problem is not the BCM. But what could it be? I have tried to find common link between all the symptoms present and they all link to the BCM.
Have checked for TSB'* and haven't found a home run there either.
Hope somebody has recently experienced similar trouble and has something to share.
Thank you

Can the scan tool communicate with all modules?

What are you using to program with?

Yes. There were a couple stored comm DTC'*. Specifically U0122 (ESC) stored in BCM and U0214 (RFA) stored in IPC. However they both comm currently as expected.

The original GM MDI

Are there any other codes stored in the BCM besides the U-codes you listed? It doesn't take much for the BCM to disable a driver for an output if it picks up a short on that circuit. Have you tested those two listed circuits for shorts?

What exactly is this current persistent symptom?

Plan to check them for shorts, yes.

Problem is that it is intermittent. Usually good. But when it acts up we lose control of wipers, cruise, IP dimming, trunk release, inside rear view mirror goes dark, "hood open" warning displays on the IPC, no radio controls from steering wheel, headlamp switch is unresponsive

It sets DTC'* ECM P0564 and P0575 and BCM B1395, B1529, B3600, B3922, U0122 when it acts up.

I think that U code was something old. It does not reset.

Your hard wired for the programming and not wifi.

It almost sounds to me like the headlight control module has an intermittent short or something and it'* causing everything else to wig out when it has an episode.