Heat not working in 1999 K1500 Chevrolet Truck?

I would check any fuses/relays related to the heater or blower motor. If the heater fan doesn't work, could be a bad heater blower motor resistor(if has one) which are usually inline in the ductwork or out next to the heater blower motor under the hood. If the fan does work w/no heat then could possibly be a bad thermostat(stuck open) or control head.