While I haven't officially sorted this issue out I believe the culprit to be Dex-cool solidifying in the cooling system causing multiple areas of the system to develop clogs. After looking the vehicle over and checking nearly everything that makes the heat work (Fuses and Relays, Blend Door Actuator, Vacuum lines, Blower motor, HVAC Controls), I have narrowed down the issue to a clog in the system. While back flushing the heater core I noticed a lot of orange gunk that resembled red clay, which is solidified Dex-cool, and am now in the process of replacing all hoses, the radiator, the overflow reservoir, and the heater core to fix the problem. Best advice I can give, if you have a GM vehicle with Dex-cool, flush immediately and refill with a 50/50 mixture of the regular green anti-freeze of your choice.