Just a hunch, but it doesn't sound like any fuse to me. Suggest you do a search here on FTE. These pickups are well known for a variety of heater problems. Yours sounds like either a PITA vacuum operated blend door failure or possibly the nefarious **** slipping on shaft issue.
Mine did the same thing.... the ole **** slipping on the shaft thing. Even though it felt tightly secured, it wasnt grabbing the shaft, and therefor no heat or adjustability.
Start simple, and then address the other complex issues.
**** was 22 dollars from the dealer.