My a suburban blows cold air in the rear but warm in the front

A friend just found out his recently purchased, used '05 Suburban had the same problem. The mechanic checked the rear "blend door", which controls the air flow over the heater core (I thought it might be a servo motor since his is all automatic) but he said that wasn't the problem. It turned out to be a thermostat. I'm not sure if there is a separate thermostat for the rear, but either of those items are likely places to start looking.