Voltage at Taillights 66 Dart?

You might want to tabulate "what doesn't" work in the car besides head and tail lights. Find the "hot" buss in the fuse panel and monitor that for voltage. See if it drops when you pull the switch

Check other stuff that should work with no key. Dome, brake, and park, tail, and hazard. You may have a major supply problem such as ammeter circuit