It appears that at one time the truck had a solenoid starter. With the foot actuated starter there was no start terminal on the ignition switch or the starter. You don't need this wire with a foot starter.
The side terminal on your foot switch is to provide 12V to the coil when the switch is closed. When the switch is open reduced power is supplied to the coil from the ign. terminal on the ignition switch, through the ballast resistor which should be on the firewall.
