There should be three wires. Black to ground, usually to a small bolt/screw on the firewall. The grey one goes to power (this is supposed to go to a power source that is "HOT" when the key is in the "run" position. The best place to get this "switched power" is right at the wiper motor, which is right there by the tach. I crimp a spade(male) type end on the grey wire and push it right in next to the power wire on the wiper motor plug. Then, the last wire (usually purple) goes to the (-) negative post on the coil. That's it, should take just a couple of minutes to wire up a tach.