Been a few years....
...but that sounds like mine at one time. Turned out to be the fuel filler door actuator that was leaking. Its next in line after the trunk, and reduced the vacuum to the rest of the system. Can't recall how i figured it out. The vacuum circles the vehicle counter-clockwise from the pump (near the spare tire in my car) and it's likely a vacuum leak between there and the drier door.
Coincidentally, I had a loose wire in my courtesy light, and it didb't work. A little solder and she's good as new. Good luck.
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)