I had that issue on my other benz: a C300. Basically water is seeping in through the cowl. The dealer fixed mine because at the time it was under warranty. But I was told then to run the ac if it should our to help eliminate the moisture.
If you suspect the cabin filter of being contaminated and don't want to renew it, you can wash it - spray with detergent - rinse and leave to dry on cloth on an old newspaper overnight - this will wick out the moisture as it will not pass air if wet
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