No Brake Lights 1969 Dart(Different than post underneath)

Maybe a lose of ground, Look it the trunk at your wiring harness and see if there is a grounded wire to your body behind your tail lights.
When the key is on you will get a feed ground.
Do your head and tail lights work with the key off ?
It could be a bad ignition switch. I am not a great tec here:happy10:
So you will get more info on this I am sure.
But I am sure the head lights and brake lights should work with the key off as well as your flashers.