Replace any of the vacuum flap actuators that you find while you are in there. One or more probably have failed by now. Replacing them later on will require "cracking open the dash", so get them done now..

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car