While it is not a 300D, my '82 240D has two vacumn lines coming from the key switch, both going to the IP valve. These small lines are hard brown plastic, and pass through the firewall near the steering shaft.

I know that the engine will not shut off with the key when the lower line is disconnected, because I accidentally pulled the lower line loose when removing the top of the oil filter housing

You may want to verify that you have both brown lines connected to the IP valve.....it would be easy to accidentally pull one loose and not notice it