This happend to mine,There is a grommit behined the dash were the heater pipes come in to the cars heater matrix. they srink with the heat off the pipes but thats only half the porblem. behined the engine were the window wiper motor is there is a water catchment were the rain , wen you wash the car , runs down and out the bottom of the car .it runs in to a funnel like pipe but this sits right above this grommit then gets in to the car you may fid the carpet lagging soaks most of it up befor you notice