Wiring factory tach?

Hey i just swapped out my 88 MM 2.6 cluster for one with a tach and non idiot gauges. The issue i'm having is i'm not sure where to run the wire off the back of the tach to.

on the stock harness i think on the left side of the truck under the hood by the brake booster there should be a a little wire thats ment just for the tac,or go off your coil

THanks, would you happen to know what color wire that may be?

i (think) it was blue maybe but just run it an check them with a metter an see witch 1 it is