After you blew air through the drains did you run water over the area to see if any is still getting inside? If it is, I think the next thing to check would be the sunroof seal. Perhaps it has lost it's integrity, allowing water to enter.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.