You may have a slight leak at the evaporator core. This will cause a build up of dust and debris like leaves and other organic matter. After time, the matter begins to decay and hence the odor.

The good news is that the evaporator core on the 190 was the easiest to replace.

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Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM