My brother has a trailblazer, and he had to put a new battery in it a couple days ago. Since then, the air and heat only works in the floor, it wont blow out the dash vents. Anyone ever have this problem after a dead battery.

 

Sounds like something needs to be re-initialized...I believe that there is a re-park procedure. Search the GM General section for "re-park" and I bet you'll find it.