No heat on S10 pickup on 1991 Chevrolet S10

Heater coil has been replaced as well as heater hoses and radiator fan runs but blows cold air

3 answers
poss stuck open thermostat. start there
if you replace the heater core already check the heater core hoses if its get hot or not maybe blocked, maybe just not working/hoses wrong connections, is your temp normal? is your thermostat new?
heater control valve broken? Temp door working properly inside?