Joe,
Looks like Mercedes knows about the lack of heat output for the drivers side!! There is a special duct being tested to correct the problem. The part # for the modification is
163-680-01-17, also your dealer should have information about this change; MB-net 109.
Hope this helps you!!

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH