Is there a white wire with a loop and a plastic part fastened to it in your wiring harness near the tach? That is the white wire referred to in the shop manual. It fastens to the back of the tach using a U-shaped metal piece which makes connection with the metal tab on the tach and slips over the threaded rod near that metal tab and a nut holds the assembly on the tach. The metal U shaped part might be hard to find though if missing.