Try to change cabin filters, also dealer sell some kind of spray to spray inside air ducks from the engine compartment, wile your air conditioning working, you probably have bacteria inside the air system
It's condensation int the system turning mildewy. Turn the A/C off but leave the blower running for the last minute or two of your drive and it will decrease the mildew smell when you start the car next time. You'll have to make this a habit to keep it from reoccurring. To kill it quickly you can use this: http://www.1z-usa.com/einszett_klima_cleaner.html
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