welcome Dan, the thermostadt controls the coolant temp. Coolant temp runs through the heater hoses and heater core. Bad thermostadt, either coolant is too hot or too cold. Spend $5 and get another thermostadt.
If that doesn't do the trick, one of the heater lines may be blocked. Feel both lines going into the firewall--they should be checked after truck warms up--one hot (going to heater core), one warm (return). If one or both stays cold, you have a clog or something. Ensure coolant level is topped of beforehand.