Ignition not working on 66 Mustang

This could be a lot of things. You're radio (and the rest of the powered things in your car) are on a separate circuit from your starter. If you are turning your key to the "Start" position on the ignition switch in the cabin, and the starter is not turning over, you need to make sure you are getting power to the starter. That's provided by a direct battery cable to the starter relay, then a smaller cable to your starter solenoid . Check that you have 12V DC at the relay... then when the key is in "Start" to the wire going to the solenoid. See the attached wiring diagram