I had the same question when I looked into hooking up a tach in my 93. From what I understood Because I have no coil (I guess) it wasn't possible. I did not look for other explanations I returned it and got my money back instead.
That was just my experience I'm sure other people know LOTS more then me
1993 Ford Ranger, Greenish, 2wd, 2.3L, 5-speed, Fram Tough Guard air filter. 2 10" Subs in the back.
Future: CB whips, Make louder, Fix Headlights.
Wishes: Bigger Tires, Lift kits GALORE, Roll Bar, Turbocharged, Baja bumper.