Classic/Antique Car Repair: 1971 Mustang Convertable wont turn off, mustang convertable, starter solenoid

Question
I have a 1971 Mustang convertible that has unfortunately sat in my garage too long.  It has a 351C, FMX Auto trans, Ram Air, power top, power windows, AM/FM radio and believed to have been built for the president of Philco Radio.  Anyway, it now starts with the turn of the key, but won't turn off when I turn the key back to shut it off.  The car keeps running until I pull the coil wire. Is this the ignition switch or any other ideas how to correct this?  Thank You. - Confused in Massachusetts

Answer
The two most common causes for this is either a defect in the ignition switch or a defect in the starter solenoid. To check, when the engine is running and you have shut off the ignition, remove both of the small wires from the solenoid. If the engine shuts down then the solenoid is defective. If the engine keeps running with the two small wires removed from the solenoid, remove the connector from the back of the ignition switch. If the engine shuts down the switch is defective. If the engine does not shut down disconnect the plug from the alternator voltage regulator. If the engine shuts down then the voltage regulator is the culprit.