When I was trying to find the leak, I thought it might be dripping from the back window. So I placed a towel directly below the back window. After a rain, the tower was dry but the carpet under it was wet. So I took off all the side plastic panels. After the next rain, I saw water trickling in along the wall directly behind the pilot's seat.

You might want to remove the side panels, then spray water above the plane and see where water comes in.

[Linked Image from visitedstatesmap.com]