My 1977 Mercedes 250 W123 sedan has an annoying problem that I cannot solve. Whenever there is heavy rain, water finds its way into the cabin and wells up on the right hand side rear passenger's floor. Please note that this is the REAR passenger's floor, behind the driver's seat - RH drive). The car does not have to be moving for this to happen (i.e. water comes in when the vehicle is stationary). I thought maybe it was coming in through the door (somehow) but I can't feel any moisture on the carpet between the door and the floor. Any ideas?