Your post referrs to an 83 380sel;On your car the Tach (TD) signal originates at the ignition module. There is a single post connector (c-116) on the side of the diagnostic connector, it should have two green wires on it.
The cable I think you're referring to (4mm grey insulation ) is the crank pickup for the diagnostic connector.The engine does not know that part exists
If your tach is in/op check fuse 12.
Good luck Mike