The tach has two wires, one is a 12v feed and the other is the tach trigger lead. The tach is grounded by the body being screwed to the cluster housing. Make sure the gauge panel has the grounding straps or is in some way grounded to the body of the car.
If you have the assembly manual it shows which wire is power and which is the tach lead. On the originals, brown was the tach lead and pink was power.
RC