Turn Signals and Brake Lights
This does not apply to my car, but some later Plymouths in the P15 era were fitted with a center stop light. The optional turn signals operated filaments on the tail lamps on either side of the car. In other words, the turn signal lights were totally separate from the brake light.
Some people would like the turn signal lamps to work as they do on many newer cars: Illuminate when either the turn signal or the brake is on. The schematic below is a simple relay system that will do this. The relay contacts should be rated for at least 5 amps DC and the actuator coil should be rated for a nominal 6v.