I have a 99 f350 7.3 the ac will not switch out of the defrost setting reguardless of where i set it. it will change from heat to cold though. Im not extremely technical but i am mechanically inclined. i searched around the forum but could only find fully broken ac issues. if i missed it i apologize.
Not exactly sure on the '99 but check on the top of the passenger side inner fender. There should be a black box looking thing. There are vacuum lines coming out of it. A mouse ate through mine on my '06 and caused the same issue. It was an easy fix.
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