This is the result of one night of rain with the Custom Cover on the airplane. The best I can tell it is coming in through the 2,300 pop rivets on top of the wings and fuselage. It then comes in through the inspection plate above the left seat.
Without the Custom Cover it's much worse and will go down under the seats can get into the wiring and into the floors.
Custom Cover goes over the wings and tightly covers the cabin and helps.
If rain is in the forecast then the seat bottoms and carpets should be removed otherwise they will be soaking wet when you get back.