The newly introduced Ford Mustang had already sold thousands of copies by the time the 1966 models rolled off of assembly lines in late 1965. Rather than ruin a good thing, Ford left well-enough alone and made only minor changes to the 1966 models. Early Mustangs are over 40 years old now, so replacement of mechanical items, such as the ignition switch cylinder, is expected.
Insert the ignition key into the existing ignition lock switch on the dashboard near the steering column.
Turn the key to the "ACC", or accessory, position.
Straighten a paper clip, and insert the end of it into the tiny hole directly below the key on the face of the ignition switch.
Turn the key off, and pull the ignition switch straight out of the dashboard.
Insert the key into the ignition lock switch to be installed. Insert the switch into the cylinder until it is fully seated.
Turn the key to the accessory position. Turn the key off, which should secure the switch into place. Remove the key.
