1973 dodge dart brake lights don't work

First check for certain that tail and turn signal lamps work normally. Make certain the rear sockets are grounded

Access the brake switch wiring and or the wiring at the turn signal connector. Get a wiring diagram. I believe the switched brake voltage from the switch is white, goes from there INTO the turn signal switch, which routes it to the rear lamps. So if you have brake lamp power (press the pedal) on the white brake wire right there at the TS switch, and nothing at the rear, it's a bad TS switch, assuming the turn signals work left and right otherwise