Believe that cars in your era up thru 80 or 81 had the so called "dreaded servo". This is a plastic box sitting above or slightly inboard of fender well on passengers side. This item has a lot of vacuum lines and heater hose entering / exiting. Do a search for A/C servo or possibly ACC servo, should find all kinds of info and tips.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150