You should have three wires going to the coil. The fat one for the dist cap, and two smaller ones. One of the smaller wires will be positive, and the other negative. You want the negative wire. If it's not marked, just try one. if it's the positive, the tach won't work, and nothing will be harmed. Just swap it to the other one and it should work.
siad nothing bout tach on the coil but we finally figures it out. and there are 5 wires. my dist. wire then 4 others but with a litlle cuttin and some butt conecters we finally found the one and it works great