You are correct that you need to have an additional wire. I just did this conversion a couple of months ago myself. I will look in the book for the correct position and connector, but IIRC it was pin 6 of the big connector. The wire is either black or black with a red stripe. I unbundled the harness in the dash, found the wire, tapped into it and used the correct wire from the harness I pulled from the parts truck I had that also had a tach at one point. Best place to get the wire with terminal is off another harness. Be sure to insulate the connection and rewrap the harness afterward.