Another way to knock out mildew from your vents is to do the following.
-turn recirculating off, ac on low, fan speed on the highest setting and make it blow out of the floor boards and dash vents.
- then spray Lysol into the intake ducts at the base on the windshield and allow the ac to run for 10 minutes.
That should knock out any nasty stuff you have growing in your HVAC system. You would still need to clean the drain if that is clogged too though.