do you remember how it was wired? I just picked one up but the wiring doesn't match other tach color coding.
If yours is like mine, it has two thick wires (blue and yellow) and three thin wires (black, green, and red).
The two thick wires are for the instrument light. Blue goes to wherever your other instrument lights go to positive, presumably either at the headlight switch or at a separate instrument dimmer control. Yellow is ground.
The three thin wires are for the tach itself. Black is ground, green goes to the negative side of the coil, and red goes to keyed positive from the ignition switch.
All of the above assumes your car is negative ground. I'm pretty sure this tach won't work with positive ground.
By the way, if you open the outer chrome housing you'll find a rotary switch on the back of the inner plastic housing for selecting between four-, six-, and eight-cylinder four-stroke operation, so make sure it's set correctly for your application. I'm running a three-cylinder two-stroke, so for me everything works out perfectly in the six-cylinder four-stroke position.
1982 MG Metro 1300: IOE 2015 Pacific Northworst GP, Longest Distance 2010 Cd'L Box Wine Country Classic
1980 KV Mini 1: Worst of Show and Fright Pig Supremo 2009 Concours d'Lemons
1978 H Special: Second-Round Elimination 2010 Lemons Pinewood Derby at Sears Pointless
1967 SAAB 96: IOE 2012 Pacific Northworst GP