Thats mildew, and its caused by water staying in the system. most likely what you are doing is using your AC and then shutting off the car with the AC still on. This leaves the ice inside the evaporator that then melts and mildew starts feeding on it every time the car is shut off. when you turn it on you freeze it all back up and the smell goes away. What i find helps is turning off AC a mile or so before destination so the ice melts and you are still blasting air through the condenser and evaporate the water. Since the mildew is already there though, you need to get some anti mildew spray and spray it in there and hopefully it will get rid of it. The drain tube comes out right by the front subframe on the passenger side. i can't remember if in front or behind it. So check there if you can see water dripping out after having AC on for a while and then turning it off.
I haven't taken that part of my car apart yet, so i'm not sure how easy it is to access from under the dash but spraying the mildew cleaner right into the condenser area will provide you with the best results. I have previously just sprayed it into the vents with so-so results.
Last edited by estomax; 05-30-13 at 08:24 AM.