The line should be brown, and it comes out of the firewall on the drivers side and connects to the back of the IP. Disconnect it and and try to start your car. If it fires right up that would indicate that the vacuum switch on your "ignition" (where the key goes in to the steering column) is bad. That black line you are refering to is a vent line most likely and doesn't connect to anything as far I know.

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Rich B

1983 300D Turbo
205k

Excuses are crutches for the weak