On my 1987 300D Turbo there is a small valve under the rear passenger's seat cushion on the driver's side. When I was having vac problems, I was able to release the headrests by pulling vac on the plastic line leading to the rear headrests. I'm not sure if yours is similar, but worth a try.....

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1960 MB 190Db w/ full sunroof
1985 Toyota Pickup
2006 Honda Odyssey EXL R&N
2001 VW Jetta GLS TDI