I have a 2012 Kia Sorento, which last year had an issue where the condensation drain line was stuck behind the exhaust manifold, causing the line to be pinched and condensate to accumulate to such a degree that the inside of the passenger floorpan was soaked.
I resolved the issue by pulling the tube out onto the other side of the manifold, and a ton of condensate drained out. I threw a fan in the passenger window, and some baking soda on the passenger's side floor pan to help "absorb" the smell.
My wife loves to use recirculate mode (Max A/C), but whenever I'm in it, I shut it off. I also replaced the cabin air filter after I drained out all the gross water and verified the A/C operated as expected.
I realize any one of these would explain a mildew smell while using the HVAC, however I noted that when the car sits for a day or two (it's hot here, like 90 degrees during the day), and we open it up... There's a mildew smell that is a bit overwhelming.
Any ideas how I can solve this? I feel like it is related to what happened last year, and that somewhere in the system there must be some sort of mildew/mold. Not sure how I can mitigate it short of pulling all of the vents out of the vehicle.