Do you have an EXT? I have to assume you do. Your rear HVAC unit temp actuator may be dead. Have you ever heard a clicking in the right rear of the vehicle that goes away in 5-10 minutes after start-up? That's usually a clue.
I would remove the rear trim panels and look at the HVAC unit underneath, and see if the TEMP actuator moves when you move the control knob. If not, replace it. Pull the battery negative cacble while you do it, for safety because the blower speed control module has constant battery voltage on it, and also to force a system recalibration when you reconnect the battery.