1974 Duster /6 rear lights

I'd start with rear body wiring connector behind the drivers side kick panel. Unplug it, and using either a light tester or volt meter, check to see if you have power at the terminals when you step on brake pedal, or use signals etc.

Personally though i think it sounds like your turn signal switch is fragged. Your signal and brake lamps run thru it. But that wont make your running lights not work.

You can pick up a full color 11x17 laminated wiring schematic for your car from www.classiccarwiring.com about $22 well worth the money spent. I have one for each of my cars 67 and 69 cuda