Follow David's advic, on the W124 in the same location, behind the glove box. Hitr the button. If it is working you should noticew an almost immediate change in the sound of the air flow. b If this isn't happening, best bet is your vacuum actuator has stopped functioning.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.