4 spd neutral safety switch

The needed wire is not likely in a generic harness. It's very simple.......it goes from the blank spade on your start relay, through a separate grommet through the firewall, and to the terminal on the clutch switch. It grounds the starter relay when the clutch is depressed.

So far as "where" this grommet goes through the firewall, perhaps someone can put up a photo. Nothing I've seen in the last 20 years LOL has one

This actually works the same way as an automatic.......the NSS switch on the auto (center pin) grounds in P or N