I had the same problem last summer on my 70 Challenger. Everything worked except the brake lights, though with memory one of the turn signals flashed kind of slow.
It turned out to be the turn signal switch, brakes are wired through it so one side goes off when that signal flashed.
There was a slightly melted portion of the switch, preventing good contact.
I bought one of the readily available 71 - 74 switched, and drilled out the rivets for the switch portion, and put the flasher guts from my 70 switch inside. Used tiny nut and bolt from a hardware store to put it all back together.
All works fine now.
Mark