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I assume your heater setting is electric control, if so either your defrost and floor heat switch failed or wire problem.
For the air not hot, check the heat control valve on the top of the hose it should be in front of the firewall. I don't know your car is using mechanic valve or electric control valve. Anyway check there first.
If I remember correctly the mode door is vacuum operated. Check to make sure that there is vacuum to the control head. If not is is possible you knocked the vacuum line off when replacing the heater core. The default position for a vacuum controlled mode door on just about every vehicle is the defrost position. This is so if there is a failure in the system you will at least still have a clear windshield to see through.
Sounds like the heater core is leaking coolant. If it is leaking it will blow steam out of defrost vents and fog the windshield. Symptoms: low coolant level in radiator, steam coming out vents when engine is warmed up, odor of anti-freeze coming out of vents, wet carpet in floorboard near heater blower motor. If so, replace heater core.
Heater flap works on vacum lines. My jimmy had same problem. Found out flap broke inside heater core. Gmc had a default. If something breaks it goes to defrost as safety precaution. Or a vacum leak in a hose to heater.
Your going to have to replace the switch. The switch it self may have gone bad, since it works on a vacuum. It's common for the diaphragm in the switch assembly to go bad, if it's not the switch. Then it's going to be a bad actuator door and will need to be freed up. I have found it to be the switch it self get dirty and the vacuum needed to activate the actuator door is not there.
Good luck and keep me posted. When you turn the switch from defrost to heater, do you hear a air leak??? If you do then it's a good chance that it's the switch it self.
does the vents swithc from defront to vent and floor or its it stuck on defrost? if its stuck on defrost then open hood check for broken or cracked vac line to hose on the hood. if it switches then remove the blower resistor and inspect the evap core for clogging if it is clean it to restore the flow of air thru heater system
I had problems with a similar truck,a 1998 GMC diesel after checking to make sure the control cables were operating the blend door and blowing the mess of the heater core,I replace the thermostats.Still no heat turns out there was a small amount of debris collecting in the fitting for the inlet hose at the thermostat housing after removing it and blowing out the lines the heat was fine.