Well, by golly, I think Brian was right about the sunroof being the leak culprit. I took a pitcher of water out to my car today and carefully poured water on the seals of the closed sunroof. I watched as the water disappeared into the seams, then I later saw a bit of water on the floorboards again.
Now, in order to fix this, I need my sunroof to actually work! I took out the motor, which didn't function, and proceeded to take it apart. There was a small black relay inside the motor, in circuit with one of the brushes. I opened it up, cleaned it, and put it back together, then did some re-soldering. Now the motor works. Geez, the things one must do just to fix a leak...
1995 E300 Diesel (Die Blau Frau)