Is the airflow controlled with vacuum using vacuum pods like the W126 models? I am not familiar with the W140 body style but I would start there since you do not have proper airflow anywhere. Or you can also look for any loose or broken vacuum lines.

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6