75 tach wiring for 77 dash cluster

Gentlemen, I purchased a factory tach from a 75 Monte Carlo and I am installing it in my dash cluster for my 77 Monte. I know the 77 wiring bundle doesnt have the wires for the tachometer because it wasnt an option in 77. What I need to know is which terminals on the back of the tach are the positive, the ground, and the tach lead to the distributor. I have researched my wiring schematic and found there is one path, for one of the poles on the tach, in the printed circuit board that doesnt have a corresponding pin in the wiring harness connector. The other two termninals have paths that mate up to connections on the wiring harness connector. Those two, I believe, are the positive and negative. But Im not sure which one is which, and the other terminal, I believe, is the tach input to the distributor. Im hoping i can use the terminal prongs for the printed circuit board on the two that have paths and thereby make my pos and neg connections and run a single wire from the other unused terminal to my distributor.  Please correct me if Im wrong as I dont want to hook this up and fry my wiring bundle or have a fire.  thanks  BILLY