Please help

I put my tach on today. I hooked up all the wires but 1. There is a total of 4. White=light, Black=ground, Red=power, and Green=rpm. I have to hook up the green one to the negative side of the ignition coil. I don't know where that is. Can anyone tell me?
I got a 87 b2200

just hook it up to the distributor should be a pos.and negative on there at the coil,that should help you bro

on the coil there are two places where the wire hook up the are maker with + and -...clean it up and fin the one you need

Use a volt meter. Ground one wire of the volt meter and tap into the wires at the distibuter with the other wire on the volt meter. Set is on AC current. Rev the engine and look for the voltage change. When you find one that goes up and down with the rpms, thats your wire. It's not like there are a whole lot of wires there, so it shouldn't be hard to find. I have never done it on my truck, so I can't tell you what color the wire is.