64 Dart headlights, gauges, interior lights went out

Sounds like multiple problems. Trouble areas....Bulkhead (firewall) connector;
Headlight switch and dimmer switch; back-of-dash connections; fuel gauge (when it goes bad, many times it takes out the temp gauge also); The drivers' side kick panel connector; trunk wiring damaged by unsecured cargo through the years; Door switches corroded will cause the interior lights not to work. They clean real easy with some find sand paper. Hit them (and other connections out in the weather) with some good lithium grease to seal out moisture and keep the corrosion to a minimum. Get a good wiring diagram, a test light, and take your time. It's easy if you think about it a little. Think of electricity as you would a plumbing problem.....it flows from one side of the battery to the other through different components and the wires are like pipes. Just be carefull to disconnect the battery before messing with connections or unfused leads that might get shorted accidentally with a careless tool.
Have fun, Mark