Your tach should have 4 wires:
red, black, green and white.
The red wire from the tach goes to a 12v source that is powered through the ignition switch (preferably to the fuse box). The black tach wire goes to ground, and the white one goes to a lighting source, like splicing it into the ashtray lamp.
The red wire going to the coil is probably positive, but just unplug the coil, and shine a flashlight into where the harness plugs in. There will be a + and a -. Which ever one is negative get's the green wire from the tach.
Take care,
-Chris
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