Why is the heat not working?

these are natorios for getting plugged cooling systems. remove the radiator cap off the radiator (only while the engine os cold) and look down into the radiator. it should be clean, the color of the coolant should be red/pink. if you see any sludge instead of coolant this is the cause. what happens over time the red coolant with the presence of oxygen will cause electrolisis in the cooling system and this process with turn the coolant into a solid. the heater core, radiator, thermostat, all hoses will have to replaced & the engine will need to be professionaly flushed.
I hope i'm wrong for your sake. this can be very expensive. I've spent 33 years with GM and have seen this ruin too many vehicles.
there is actualy a class action law suit against GM for this issue. I don't know if the time period has lapsed or not. something to "google"