[hotrods1795]

I bought a 69 Barracuda that came with a painless wiring system and it had a tach in it with 3 wires. The tach I'm putting in has 4 wires and is an Autometer Carbon Fiber with a shift light. I know one wire goes to the lights, one to the ground and one to a 12 volt source. Where does the other wire go? Could someone help me to get this tach to work. There is also a 6AL MSD and blaster coil.

Thanks