2003 chevy truck Air Conditioning Issue

I have dual climate controll but the driver side blowes warm air all the time. The passenger side works fine. Is there a controller somewhere and how can i get to it?

http://www.freeautomechanic.com/airconditioning.html

There is a vaccuum door in the assemble that controls cold/warm air mixture. Either the door is stuck to the heat side or the diaphram is bad. Located:Behind the dashboard you will see on the assy a thing that looks like a pancake with a vaccuum line attached to it there is also a rod that you cant see going into the heater box.