I would seriously load test the system (battery) and look at alternator....

This is a 3.9L v6? is the alternator up top? Can you put a screwdriver on it and listen at the handle? Bearings whining?


This could come down to something ridiculous as a stuck power seat adjustment switch...

 

Local mechanic loaded tested yesterday, Stated he got alternator to put out over 17 volts. It don't make any sense.

 

This can be caused by a multiple issues. My Camaro had a trunk light that would stay lit when the lid was closed, BUT not all the time. Running it every day would keep the battery charged but over a couple months would gradually lose the fight and not start. My 2002 Tahoe had a bad instrument cluster. The dealer had it for about 3 weeks trying to find it. They said the cluster always has a parasitic current draw to keep things alive, but the cluster in my Tahoe was about double what it should have been. After replacement of the cluster, (knock on wood) I never had the problem again. A sensitive ammeter to see what the current draw in the vehicle is when it is off is probably the only was to find if that is indeed the problem. Once it is determined you have a high parasitic current draw, it's even worse to find out what's drawing the current since it is usually so small. I've fulled a fuses systematically to find what circuit is causing the problem.

 

Installed new battery saturday. Left Winnie, TX Sunday. Drove to Big Springs Tx stopping over night in Fort Hood area, about 500 mile trip. Monday left with car to go up town and radio wouldn't play nor sound from turn signals. Found out today that turn signal sound comes thru radio. Turned around and parked car. Talked to local Chevy dealer today. He said to unplug the mirror and that would disable BCM along with onstar. Car would not even turn over today. unplugged mirror and turned ignition on, radio played and car cranked up and ran. Everything works fine now. Undoubtedly that circuit has a short.