Seems to me that just a few weeks ago there was a guy in New York(??) who had a similar question on a 126. After the usual bout of cleaning drains and running water over the are it was determined that the pipe stub that the drain tube attaches to had rotted out. Fix was either the body shop or a glass/window shop job.
A search on "sunroof drains" should bring that up as a recent question.
'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.