I hate to do it, but I am going to have to opt out. I am having a technical problem getting logged into my technical sites. I will get back to you asap if no one else has not. Sorry for the delay.
Carhelp4uhere asked me to help out here.
I found a TSB on the alternator being mis-stamped with the wrong Amp's.
Do you know the amp output of the alternator thats in it?
It's supposed to be a 145 amp alternator. This is what might be giving you trouble, especially under a heavy load..
They had 2 different versions in 2004. The 105 and the 145. I have a basic package without onstar or navigation.
I had this serviced at the dealer in April, so I am assuming they installed the proper alternator and battery.
I'm afraid the only thing that i can think of would be that it blew a diode and causing this. I recommend having the dealer check it, it might still be under warranty. Dont worry about the except if you dont want to because i sure cant tell you without a doubt where the fault lies without being there to test.
Best of luck my friend!