I can almost guarantee it's the vapor barriers leaking. Given enough time, even without work being done on it, the glue will seperate and let water in. It happens on E39's too. Pull the panels and look at the bottoms of the black vapor barriers. If it's not obvious, close the doors with the windows up, and run water down the glass for a minute. Open the door and look for a trickle of water getting by the black glue.
I see it all the time.