I have a 1987 Chevy 1500 Truck and when I turn the heater on there is only cold air that blows out even when it's on Warm setting. I checked the radiator and it's filled to top. what could the problem be? Thanks.

diverter valve on heater core [hose] going from waterpump to firewall vaccum line off or valve sticking

also posible heater core is pluged when the truck is running do both hoses feel hot if so cores good and diverter valve is working