No heat from blower

1990 C1500. No heat from blower. Fan works, thermostat seems fine. There's two lines on the top of the rad that are both hot after warm-up - a big one from the therm and a smaller one on the other end going to the core. The bottom big hose from rad to pump is cool, though. I thing it might have to due with actuator or door since I no longer hear the actuator when I adjust the heat, or it's quieter for some reason. Ideas on how to proceed?