1993 Plymouth Acclaim…this grandma car just keeps running but it has no interior heat. Coolant is full, no overheating so thermostat’s okay. I had a old Corona and the petcock was full of gunk so no hot coolant entered the cabin. Is this likely the same thing?
Just b/c its not overheating doesn’t mean the thermostat is ok. It could be stuck open (rather than closed). Yes, it could also be a problem with the heater core, but I’d check out the thermostat first. Test the actual coolant temp and compare it to specs to see if you’re running too cool.
I also am assuming that it is actually blowing air and there isn’t just a problem with the fan or control doors/ducts.