Its been a heck of a week for me and truck problems were the last thing I need but here is the basics.

Vehicle : 2004 Chevy Avalanche.

Situation: Friday, I wen't to lunch with a friend. when I turned off my truck the air condition continued to run. I turned the fan off but nothing happened (fan continued to blow, even turning the fan down didnt seem to change the intensity of it). I parked it for about a half hour over lunch, and then drove it home. When i got home it was still going, and when I tried an hour later, the battery was dead.

I wasn't able to get someone over for a boost until today, but the battery wouldn't take a charge enough to turn over.

I'm not very mechanically inclined, so Im not sure what to expect. If I toss a new battery in there, I assume the same thing will happen.

Question: What is the likeliest cause of the fan not responding to the controls in the vehicle? What will be the best way to diagnose this?

Thanks for any help or advice you can give.