I think the real problem is when you shut the car off the air ducts close and with the cabin filters on top, you seal in all the air and moisture. A fellow BMWCCA member who lives in Sierra Vista, AZ (we use a lot of AC here in AZ) says he has permanently licked the problem. He wired the damper in the duct part way open, so it will never fully close. He claims it works flawlessly. I haven't got around to getting in there, so I continue to spray mine through the hole I made next to my gas pedal, every month or so.
chiefwej 2003 540i ///m-tech 6-speed
Black Sapphire Metallic w/gray