I would check the negative cable at the battery and the load management gizmo which hangs onto that cable. I will search for a post I did years ago where I cut and pasted the info on the load management/charging system from the repair manual, which is quite sophisticated. Somebody flamed me for posting useless information, so, shouldn't be too hard to find
EDIT:Starts at post #12. Avalonandl was not a fan
https://www.camaro6.com/forums/showthread.php?t=596073