The climate control system is powered by vacuum. If there is insufficient vacuum to operate the system, it defaults to full fan blow out the defrosters. This is a very strong indication that you have a vacuum leak somewhere in your truck.

The most common place for this leak to occur is the ESOF hubs, which are also vacuum powered. Check the hoses leading to the hubs and the hub seals. The second most common cause is a failure of the vacuum solenoid, which controls ESOF and the heating system. Third most common area to look is all the vacuum hoses in general, look for rot, tears or a hose that just fell off.