The tach signal comes from the gray wire at your engine. Depending upon the year and model number, it could be connected to one of terminals that also has a yellow wire on it at the rectifer which is on the electrical plate on the left side of the starter solenoid. If your engine has a newer style rectifier/regulator then it should have a gray wire on it that is connected to your tach.
Tachs get their signal from one of the yellow leads coming from the stator. The connection is normally made at the rectifier since they also feed it. A shorted rectifier can keep your tach from working.
