Down here in Miami, this happens to many different type of cars. The humidity might have something to do with it along with running it on recirculate all the time. I know the manual on my charger and my old BMW X5, clearly stated that the recirculate mode should only be used to cool the car down when really hot, and should not be used all the time. On the ram with the auto A/C, just let it run, set it and forget it. The A/C will run (in auto mode) in the recirculate mode to cool off and then run normal, even if you don't see the recirculate light on. It will switch all by itself.

 

In Texas you have to run the recirculate mode or the damn truck will never cool off in 104 degree heat. I try to run the recirculate mode long enough to get the truck good and cold then switch it to fresh air. The problem I have is when it's 104 out and you are just running around town the truck won't cool off on anything but recirculate mode.

 

Put the a/c on recirculate and spray some antibacterial spray like lysol or Febreeze in your cab about once a week. I always use recirculate and I do this to prevent the kids from getting any germs or bacteria that builds up in the truck. I also do this in the house, its good sanitary practice.

 

Seems like it matters whether I park on level ground or not. If I park on the steep part of my driveway, it has a little smell. If on the level part, nothing. It must not drain properly when parked on the steeper angle.

 

Mine has done it. But only occasionally, not all the time, and it seems like the only times I did notice it were after I parked it in the garage (which doesn't smell btw). I don't really know if it still does it or not, I ended up putting an airwick clip on air freshener on one of my vents to just make it smell pleasant all the time, and I've never noticed the mildew smell since. I forgot all about it actually.