Heater/defrost fan keeps running

[CaliLivn]

This is for my dads 74 pickup. It has 350, AC an everything. The other night when he got home he turned off his truck an the heater fan kept running. He started his truck real fast an shut it down an it still kept blowing air. He got nervous so he popped the hood an unscrewed the battery cable an it shut off. In the morning he reconnected the battery cable an it started blowing air right away. He tried starting his motor an shutting it off an removing the key an it stayed on.... Finally he pulled the heater fuse an it stopped. Has anyone had this happen before? I would like to try an fix it myself before taking it to a shop. Thank you