Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
There is an actualtor on the left side of the heater box. Would be level to about where your knees are when your driving. That is what controls the temperature blend door. Here is what it looks like;
Sometimes you can tap on them and they will start to move again, (like the one in my girlfriends 99 pontiac) A new one would have to be bought from a GM dealer and they are about $85. Being it is an electronic part, I don't recommend using one from a salvage yard, thanks