a friend of mine who's a bmw SA told me to do this once in a while:
Before you start driving, turn on your car, and turn the heater on all the way at the highest temperature with all the doors and windows closed. Then wait a few minutes and get into the car and turn the AC on. He said the high temperature helps in reducing the mold build-up. It won't completely get rid of the mold, but it helps a lot.
It could be an annoying procedure but you don't have to do it everyday, just once a week or something until the smell is improved. I've done it to my car and I've seen a huge improvement after a couple of weeks of doing it. Give it a try