Water leak entering rear seat area floor

Try as I might, I've not been able to locate a pesky leak that enters and pools in the area under the seat seat cushion as seems to wick its way onto the right side floor area where a rear passenger rests his feet.  Any recommendations on how to trouble shoot leaks... nothing I tried has worked yet.  There does not seems to be a leak around the door rubber mouldings.  However, if the car is left outside in our rainy weather, the inside windows get all fogged up which suggests to me that there's moisture in the car that doesn't be there.