After you clean it you can spray some Lysol down the vents and make sure you turn on the fan. The mold smell is usually when you drive with the AC on and then turn the car off right away, the moisture that is trapped in the vents turns to a mold smell. If I know I'm done with the drive and I have the AC on, before I turn the car engine off, I turn off the AC and open the vent and put the fan on high for a few minutes. Never had a problem since I started to do that. On the smaller import cars, the vents have more angles to make them fit in smaller places than pop's big'o sexy American car. These angles have a higher propensity to trap the cold air.
good luck