I have a 2004 GMC 2500HD 4x4 with the 8.1L. The heater only is hot on the passenger side. It has the dual climate control. It also doesn't change temp when the A/C is on . It blows full cold and won't change when adjusted on drivers side. I am assumeing it is a temp door somewhere under the dash. Is this a pain in the ass to fix? I have been working on cars and trucks all my life, but I never messed with this kinda stuff. Any help would be cool!! Thanks in advance!!