It is also possible your drains are plugged. There are some drain tubes that carry water from condensation or any other source that might make its way into the HVAC system, like rain water coming in with the air. I believe these drains are located on either side of the transmission under the car. Left plugged they will contribute to a musty smell developing in the A/C system. Try a search on the subject if you think you want to explore this further. The drains are discussed relatively frequently. Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)