I cannot diagnose your truck from here but I can tell you how it works and you can check it out yourself.
The heater control valve which regulates hot coolant flow through the heater core is actuated by the air inlet door vacuum.control motor circuit. Vacuum at the heater control valve shuts off the coolant flow. Check for vacuum at the heater control valve. If there's vacuum and the coolant is flowing through the heater core producing hot air, then the valve or the linkage has failed. If there's no vacuum when the knob is set to 'cool', then there's a malfunction in the dash control assembly.or a failure in the vacuum supply to the dash control. you'll have to trace out the malfunction. You should be able to see the control valve lever move when vacuum is applied and released. It's possible that the heater control valve has failed, it's possible that the dash control has failed, and it's possible that the vacuum source to the system has failed. There could be more to it but the problem probably is fairly simple, you'll have to delve into it to find out.
A vacuum gauge and vacuum hand-pump are useful tools for this diagnosis.
Here's a directory link for you:
http://repairpal.com/mazda-repair-in-rocky-point-north-carolina
