The factory tach needs a switched 12v power source. Usually a pink wire will be switched 12v power. The tach feed circuit goes to the negative side of the coil as mentioned prior if you are running a points distributor or the "tach" terminal if using a GM H.E.I. distributor. The tach metal housing needs to be grounded. Mine had a metal type strap that would have grounded the clock to the cluster if I remember correctly. The tach lights can be connected to the gray wire that is from the headlight switch to feed the dash lights and can be dimmed with the dimmer.
That wire should be in the harness close to the clock was. Sometimes it will be tapped to the harness but since you already had the clock option just reuse it. The orange with black stripe circuit is a constant 12v power source. Don't connect that to the tach.
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1971 Pontiac GT-37
Car is a junk yard dog and maybe one day will be restored.
