The smell is likely to be from mildew growing in the HVAC vents.
I suggest that you do the following:
Spray a goodly amount of Lysol spray disinfectant or a specialized HVAC sanitizer (Frigi-Fresh is one brand) into the HVAC intake which is located outside, at the base of the windshield. Turn on the HVAC fan in order to pull the mist through the ducts. Repeat in a few days.
If this doesn't help, then the only solution would probably be to disassemble the dashboard in order to access and remove the ducts for a thorough cleaning with bleach. However, this would be a VERY pricey proposition, based on the amount of labor involved.