Sounds like your blower works, but no heat?

My '90 3.9 Dakota would not product heat brand new off the dealer lot. Defective thermostat was stuck open. New thermostat got the heat back. On a higher mileage truck though, I'd agree an air bubble or heater core blockage.

I did have a friend that had a F250 diesel with no heat. His problem was a big air bubble in the heater core, system had to be burped, then he had heat.