The air filter exterior box is under the hood, right in front of where the front passanger's positioning is. if you open the hood, it's covered and nudged up near the front windshield. just pop open the 2 clips, and remove the cover...then remove the plastic grill(easy). Once you remove the plastic grill, you can use your flash light to see, but the box is about 8-10 inches deep and has a small rubbery drain area. use something to clear it out (mine was filled w/ water to the rim, so I reached in and used my fingers and wiped away some fine dust/mud).
Keep in mind, this is different than the interior cabin charcoal filter, which I think is under the glove box. this is connected to the exterior air box - which is why when the drain plug is clogged, all the water automatically comes into the cabin. to me, it's a pretty obviously design flaw.
bottom line, this simple drain plug (that didn't drain) is common for MB techs in TSB, especially for damaging the blower motor.