Nissan D21 Z24 Z24I Tach Wiring help!

I need help wiring in a tach for a 86 nissan d21 pickup with a z24 engine. It was originally a z24I I'm not sure if someone swapped motors or just changed the intake to a carb model. I want to add a tach but cannot figure out were the negative tach wire is. Does it matter which coil (Exhaust or Intake)? Do the eight spark plugs make a difference on a 4 6 or 8 cylinder setting? I know someone has does this mod before, so someone knows what I am doing wrong. I have tried two different wires and at my wits end for something so simple. Please Help!

If this is a repost of a thread I apologize, but I have searched everywhere before posting.