Had this happen on my 300 TDi passenger side - above your feet and behind/below the washer bottle is a main loom entering the cabin through a grommet - it hadn't been fitted properly, water gathers at the bottom of that slope and pours in,worst part was removing the washer bottle to get at it. also try the mud flap attachment angles under the floor, the bolts had rusted and made the holes bigger, as I had side steps and no flaps i removed the brackets and did a tinkers tin pot repair - nut and bolt with two penny washers.