[Zappo]

Or else if everything else fails here is a desi jugaad to find the source. Since you say it happens only when you wash the car we can probably be sure that the washing water is seeping in. Do something, ask someone to sit inside the car, doors locked, glasses up. That is to say the way your car normally is when you wash it. Now take a pipe, bucket or whatever you use and start the wash routine. Ask the person inside to keep a watch. Once the carpet starts getting wet the person inside will also know from where it is coming in. Whether its the door beading or the window glass or some plug on the floor.