This is a very common problem. Manifests itself as a puddle in front and rear foot wells, most commonly on just one side of the car to start with. For quite a while Mercedes were fixing these for free, but that seems to be getting rarer now, and in fact a lot of cars are coming in for their second time. Air vents behind the rear bumper are the root cause of this fault. Mercedes have superseded the original part number, but to be honest I cannot see any difference in the part, so it will likely rear its ugly head again in time. First step is rear bumper off... You can see the two vents just below the black bumper reinforcers. This is with the vents removed... This is the vent itself. You can see the yellow/cream soft rubber seal. This is the problem. It stops sealing and rainwater sneaks in between the body and the seal. From there it tracks along behind the inner arch trim and sluices down under the rear seat onto the floor. The design is not that clever. The rear lamp seals to the body with a triangular foam, and rainwater drains behind the body of the light itself, through the back of the bumper reinforcer and straight onto the yellow/cream seal of the vent. To fix the problem for the longer term I fit some new vents, but clean the mounting area thoroughly with some degreaser... And apply a bead of silicone sealer to the yellow/cream edge of the new vent... With the new vent clicked into place a satisfying splodge of silicone oozes around the edge. It matters not how it looks, because the bumper hides it all... All that's left to do is refit the bumper.