It sounds like the alternator/starter is going out and needs to be replaced. but to be sure when the light is on check voltage at the 12 battery to see if its above 14.5 volts when running. If its down at 12 volts is no good. I would suggest an AC Delco Rebuild from RockAuto.
Here is a guide so you can test the alternator with diagrams below on how to change the unit out. You must disconnect the hybrid battery and the 12 volt battery as well which I have given instruction for as well.
I also have the alternator wiring diagrams so you can see how the system works. Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.
Images (Click to enlarge)
Friday, January 18th, 2019 AT 1:47 PM