Heater vent control

Car: 1992 Dodge Dynasty, 3.3L, A604, 160,000 miles.
Just noticed today that no matter which button I push on the vent control, heat only comes from the defroster outlet. I no longer hear the vacuum being
relieved like I used to when I changed the settings, nor do I hear the vents opening / closing.
Does anyone know of a common point of failure that might cause this? It could give me a starting point before I go and troubleshoot each portion in trying to track down the cause.
Thanks!
Bret
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Check the main vacuum connector at the brake booster or the check valve at the booster. The default position is defrost mode whenever vacuum is lost.
Glenn Beasley Chrysler tech

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Thanks, that will be the first place I look.
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