Can someone help me?

I have a 91 ford eb (carby)
It runs and goes but when at a set of lights with the sound system I carnt tell if the car is running so I bought a tachometer and tried connecting it every way possible under the hood but no luck. Does anyone know any other way to hook up a tacho? It has red black and green wire.... also the car never had a tacho so I carnt hook it up that way.