If by fresh air filter you mean cabin air filter, it's located inside the glove box

 

Something I find that it quiet dubious; my filter is conspicously absent.
This tells me that it either was a known problem and someone attempted a fix or it was not installed in the factory.

I am about to take my car apart from the inside out. I will try and take pictures to maybe assist others in the eternal search for "the leak".

 

are you positive the drain plug is not clogged? because I know its a diff car, but my lancer used to do the same exact thing, water would come outta that box, and sometimes it would be clogged like a inch past the firewall when i would stick a hard wire in it to unplug it

actually, I've had the same thing in my house before too, I'd hate to see you tear it apart and it be something simple, but a thing you could try, if you can find the end of the tube that's in your car, unplug it(water will probably come out if you don't crimp it) and if you have a compressor try blowing it out with air.

 

on the driver's side under the mat when there is a very heavy rain......have to check the firewall as posted earlier......

 

Take it to the dealer and have them fix it. They will know right where it is and how to fix it. That leak is std equip.

 

Guys and gals, I am in the middle of rectifying my leak on the drivers side footwell (it is almost dry now). I have definitively isolated the leak source to be the drain tube running from the sunroof to where it exits, which is a black rubber piece between the door hinges (in front of the wiring harness). Pour some water into the non-leaking drain hole in the corner of the front of the sunroof area (sunroof open) to see where the water is supposed to come out. Mine comes out where it is supposed to but also leaks inside at the door pillar below the air bag. I ran weed trimmer line down the drain tube from the sunroof, and also the reverse, to verify that there was no blockage. So I know what and where the leak is, but I have one more issue, and that is how to remove the inside door pillar cover (contains the air bag and tweeter) so I can have access to repair the sunroof drain tube. Any links, photos, etc, that would help do that would be appreciated. I suspect that many of these leaks have the same origin, which is why the dealer cannot locate them. They are looking in the wrong place! Took me only 10 minutes to verify my leak origin. Hope this helps. Now if I could just get a free replacement shifter assembly I'd be okey dokey.