Maybe this will help....if you use a carwash, it will help if you move the air intake to recirculate the entire time you are in the car wash. This will stop the system from sucking in all that wet air. May not seem important, but I have read it really helps. just my 2 cents.

 

Either that or one could argue it is similar to dust on your carpet after driving dirt roads a lot, but the dealer should do it as a favor for your future business.
Of course it's regional: you need warm temperatures and humid air mixed with other factors. For example not using AC for the last 5 min of each ride could be enough to dry up the system and avoid the problem all together. There also could be blockage of drain system making it more more prone to this problem but there is optimal, narrow range of humidity, temperature and light for the fungus to grow the most . I never looked but I bet there should be some solutions sold in auto stores for DIY.

 

One theory I have that is that this often occurs in cars that are Garage kept. When we moved into a house that had a two car Garage as a kid, my Mom's 83' Cadillac developed it's first stinky climate control about two months after we moved in (86') The car was kept outside before that and never had an odor. Since then every car we have owned and garaged has developed that smell. I think it may play a role

 

Could be something to do with a humid garage. If the AC had been running then there is the puddle of water under the car in the closed garage.

I read its best to turn the vent open and the fan on high a few minutes after starting the car. This should blow out any moisture thats collected. I would think that may be a good idea after parking the car for the night.