If your 1983 vacuum system is alike mine from 1979 I might be able to help:

A possible source of failure could be the vacuum-/keyswitch in your driverīs door. It should be removed and checked with a Mityvac vacuumpump.

If you suspect a vacuum module in your rear driverīs side door it should be removed and the rubber diaphragm should be re-glued at where the ring sits. With the help of the vacuumpump you can put it under water like you mend a flat bike tire and look for bubbles.

You wrote your system works fine up to the front door. Just make sure that the vacuum valves under the hood are intact. Again check with the vaccum pump.

Regards,

Jorg
1979 MB 450 SE
1979 MB 450 SEL 6.9
1991 MB 300 SL