You can replace this piece yourself, it is under the dash on the passenger side. You may have to remove a plastic cover, then you will see it on the hvac housing next to the blower motor. This is a VERY common failure, so have confidence it will fix your problem.
The part is approx. $130, sorry I don't have the part number, as there are a couple of choices depending on your vehicle. You will have to give them your VIN number to get the correct part.
http://www.justanswer.com/questions/1gfro-2003-z71-chevy-...