ok, long story short, i put a new under dash and a/c wiring harnesses in a guys 70 442. but i'll be damned if i can figure out where one of the wires on the back of the tach plugs into, nor do i remember unplugging it in the first place. for all i know, it might not have even been hooked up, but i know the tach worked when i pulled the car into the garage.

so anyway, if your looking at the tach from the back, from left to right.......

gray wire - light
male tab - orange clock wire
post - tach wire
gray wire - light

those 2 gray wires come together at 1 black male plug which goes to the female on the main harness. alright, i got that. but.....

on the lower right hand corner there's a.... maybe 6" long gray wire screwed to the housing with a black male plug. that's the one that has me stumped. i see no where for it to go, or anything on the old harness that it went too. is it just a ground that wasn't hooked up?

 

some one just posted about this the other day, try a search and there is some good info on where what wires go where

 

See if this helps...
https://classicoldsmobile.com/forums...up-messes.html

Particularly this post:
https://classicoldsmobile.com/forums...tml#post145636