Hey Amoretto,
The smell is probably coming from mold growing because of the moisture from the condensation form the A/C Condenser.
(Perfect breeding ground for mold; dark moist areas.) I know that there are many products out in the market that can take care of this problem. The trick is to know where to use them. I'm always for saving hard earned cash so, I try to use items I already have on hand. When I had this problem in my wife's 03 Hyundai Sonata, I used regular Household Lysol spray which kills all sorts of things including mold. (Think the wife had lavender scent), many scents are now available all for a fraction of the price of the specialty auto cleaners found in parts stores. (I think Wal-Mart sells them for about $3 a can)
Okay now for the actual instructions. Spray the air intake outside of the car, usually beneath the windshield. Please note that this is not where the smell is coming from but you spray it, because this is most likely where the spores entered the car and there may be more hanging around making their way in.
Remove the glove box and remove the cabin filter, spray the opening where the cabin filter normally sits, paying special attention to spray towards the heater core( towards the driver’s side) be generous with the spray it won’t harm anything. (I used about half a can for the entire project) Let the spray sit for about ½ hour before putting it back together. (You may want to replace the filter while you’re already there) Run the car with the heather on high along with the defrost to clear some of the smell.
But please keep in mind to use a scent that is pleasant because it will be with you for the next few weeks every time you run the A/C or Heater. (It will eventually dissipate)
I did this for my wife’s car 2-3 years ago and the unsavory odor has not returned.
Good luck!