You know I had the same problem when I bought my car last year, but it was on the passenger side. It seemed like every time I washed the car it would be wet under the floor mats. I just could not figure it out, but then I noticed that the rubber striping inside the car that line around the dashboard was off. After I put it back it stopped the problem.

(Say if you open the door, its the rubber striping that lines around the dash board right where it meets the fire wall) sorry that the only way I can describe it without pics.