My guess is a vacuum leak. But let me understand what your saying. So if you shut the trunk lid it wont hold down closed?

On my S420 the trunk lid will close but wont be pulled down to be flush. After pulling the fuse the pump worked again and pulled the lid down to be flush with the rear panels.

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Current:
05 E320 CDI
07 GL320 CDI
08 Sprinter
05 Dodge Cummins
01 Dodge Cummins

Previous
2004 E55 AMG
2002 C32 AMG (#2)
1995 E300
1978 300D
1987 300D
2002 C32 AMG(blown motor :[
1981 300SD
1983 300SD
1987 300SDL
2002 Jetta TDI
1996 S420
1995 S500
1993 190E 2.6
1992 190E 2.3
1985 190E 2.3 5-Speed