I would have to agree with mp. The rear air temp should be the same as the front.
Is it possible that the one front Red/Blue thumbwheel is connected to separate baffle mechanisms that are supposed to control the temperature for the front and the rear ducts separately yet,for some reason, in your GT, affect the front air ducts only. In other words, the rear baffle , if there is one, is stuck in the 'Cold' position and never moves to let anyb warm air into the mix.
I am guessing here not knowing the exact HVAC configuration in the GT but mp is correct in suggesting a talk with a service advisor. Then again, I learned a few things from some of your responses to others' questions for which I did not get a correct answer from an advisor at my dealer.
A diagram and description of the HVAC system would be helpful to understand how the system is supposed to work to possibly determine why your symptom is ocurring. That would not be easy to obtain.
Last edited by Fish23; 11-14-2012 at 02:35 PM.