The air conditioning only comes out the defroster vents no matter what I set the controls on in a 1997 Buick LeSabre.

Broken vacuum line most likely. Try removing the dash assembly trim and locating a small thin black tube from the rear of the control knob itself, and then trace it throught the firewall looking for melting or breaks.

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I had a same problem, I checked fuses (behind the glove box), replaced disply unit (dual air, automatic), replaced programmer with all vacuum hoses (do not ask about bruises on my hands and my frustration). Nothing happened. Then I figured out solution above. You should get better millage, too.