Modified speakers? If so, water can leak around them from the inside of the door, dribble along the inner door seal and then, depending on the inclination of the car will dribble down in the footwell. It was this that ruined my carpets, so took them out and then decided to make my GTi a track car.. removed pretty much everything from the interior and sealed up the speaker holes with aluminium flashing :) Assuming you don't want to convert your Lupo into a track car I'd advise to use a bit of sealant around the speakers when you pull the lower door card off.