Tach hook up question.

[Notplayn]

I have a GM factory Buick GN tach I want to install in my GN. The tach has four wires, two connectors with two wires a piece. I am guessing that two of the wires are for pwr and ground for the back light. Not sure what to hook the other two to. One I know is for the tach signal which I would like to tie into inside the car. Can some one recommend a place to tie into the tach signal inside the car. Or for best results should I run a wire and connect it to the white wire with the green connector behind the alternator. Finally what is the fourth wire for?