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Eventhough the heater core is not leaking coolant on the floor, It may be clogged up. What you will have to do is get the truck to running temp and check the two heater hoses going into the heater core on the firewall. Both should feel about the same temp, Be careful though as they will be very hot. If one is hot and one is not, the heater core is clogged. Another possibillity is a bad blend door motor located on the heater box behind the dash that is not allowing the hot air to enter the box.
Thank you and I know this sounds ridiculous, but I don't exactly know what a heater hose looks like. If I go to Auto Zone or a place like that, will they tell me, or will I have to go to a mechanic?