you likely have several Dash vacuum pots out of sorts. Your supply line goes to an electronic gang valve if it's the typical W124 Setup. the main line should be green... brown is generally they key cyl if I recall right. the default system with errors is Defrost

The electronic valve should be on the right hand side of the HVAC box where the glove box is, theres a retention pin in the bottom and a place where it snaps into place on the box, but it may have jarred loose, I will post several pictures in a moment

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed