Randomly and without any discernable pattern my Buick Lesabre's (Custom trim) A/C will start to come out of the defrost vents. I'll have it set on either Bi-Level, or Max, or anything, and all of a sudden and for no reason it'll switch to defrost. The indicator light will say its where I wanted it, but it will come out of the front defroster vents. I can play with the buttons and sometimes it'll come back when I move a lucky dial, other times it just stays like that until it randomly clicks back to what its suppossed to be.

Google is inconclusive. Help. Please. This is Florida and I'm fucking broiling.