The tach sense wire is gray and is connected to the negative terminal of the ignition coil. That wire looks to short to reach the coil and may be a extra wire for diagnostics. Try disconnecting the gray wire from the back of the tach and using a voltmeter see if you get a reading while the engine is running. If no reading the wire isn’t getting signal if you get a reading the tach is bad.
Tach / RPM
Linear Mode
