1975 Dart Swinger. No headlights/brake lights
If the high beams work the ground is good and headlight switch is good out to the dimmer switch. The dimmer switch changes high and low beam. If that switch is working only other fault would be 2 bad bulbs.
Brake lamp switch under the dash has 2 wires. A good fuse puts power in on the red? wire. Press pedal and power goes out on the white wire and into the turn signal switch. Confirm that much first.