I acquired this vehicle from my brother-in-law two years ago. I've noticed that when it rains, the rear seat floorboards end up soaking wet — in some cases, with standing water. My cursory examination seems to indicate the problem is much worse when driven in the rain as opposed to a static situation. I am now suspecting that the rear wheels throw water up into the wheel well when in motion and that water gets in through the body and ends up on the rear floor.
I believe this may have been a problem for him as he complained about the 'smell' of mildew in the car but mistakenly believed it to be from a repair to the rear hatch that was improperly done by the Scion dealership he visited in Los Angeles, CA. He never established where the smell came from and of course, being California, it only rains a few months out of the year, so the problem would only present at certain times. I keep getting postcards in the mail from the local dealership telling they would like to 'buy my car' – towards a trade-in of course, and I think to myself, 'I would never buy a Scion or a Toyota again.' I am going to assess the car this weekend and attempt to establish if my theory is correct and, if so, I am going to clean it up and apply some spray-on rubber to points.