Tach Wiring Help.

[Scootin 81 C20]

Hey guys, I was going through a box in the garage and I found a tach that I want to hook up in my truck to keep an eye on things. I know where the two wires for the light go to, one blue wire(-) and one yellow wire(+), but the wires for the tach itself have me confused. The two wires for the tach are one green wire and one black wire. I'm pretty sure that the green wire goes to my tach output on the distributor, but where does the black wire go to? Any help is appreciated.

Thanks
Benjamin.