Girlfreind has a 1980 300sd. About 125000 on a stuck odometer. Very Clean and rust free. Well, I'm relatively new the whole vacuum scene, but I've done enough reading and diagnosis to understand most of the discussions here. My question is, What is this switch? Why isnt it hooked up? what should it be hooked up to, if anything?

This is on the driverside firewall left of the brake booster.


I appreaciate any help you can offer.

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1982 240d, maual, 235k mi (flat black) $500-owned since, 9-06

1980 300sd, auto, 125k mi (silver)$1500-girlfreind owned since, 6-08