If your doors are operated by vacuum, they are normally spring-loaded to go to the defrost position when there's a leak or other failure of the vacuum supply. That's done for safety. Your feet might freeze but the windshield will be clear. Look for a small hard plastic vacuum hose under the hood. Be sure it's connected to the intake manifold and not cracked. There should be a small check valve too. You might have to temporarily bypass it to see if it's plugged or stuck.