That wire (yellow) is there otherwise you would have no electrical power anywhere. Take a very close look at the starter switch, the terminal stud that has the main heavy battery cable on it that goes to the battery negative terminal, its a positive to ground system as original build. Also on that same starter switch terminal will be a smaller wire (yellow) which disappears into the harness and ends up at the ammeter. If its not on the starter switch it may have been modified from original and moved to the battery neg terminal. Its there somewhere for sure. Take a look at a wiring diagram for your 36 Ford and follow the wire. The drawing you posted is not a wiring diagram as such but a picture of the harnness. Regards, Kevin.