Today, out of the blue, I turned the key to start my 66 pony. It started cranking and then suddenly (not like a weak battery) it just s th stopped cranking. I ended up having it towed home, but while headlights, horn, instrument panel and cigarette lighter appear to have full voltage, the run and accessory position have no apparent power. I'm thinking it has something to do with the ignition switch, the relay or the starter. Am I on the right track? Where and what should I start checking?