how connect tachometer?

I have a 1994 Tercel. To connect a tachometer the manual shows 3 wires, or leads I guess they're called, coming out of the tach. Two of the leads go to the battery and one to the diagnostic unit. My tach and all the tachs I've seen only have two leads. Where can I get a tach with three leads if I do indeed need that many to check the idle speed? My car does not have a built in tach on the dash. Thanks.

two go to battery, one for it to work, and one for the light. the other goes to the blue strip wire in the diagnostic unit.