More info.
From fuse to lamp, there is one continuous wire to power the Left hand tail light.
The right hand tail light wire is spliced into the continuous wire, at rear of vehicle.
The tail light voltage, stop light voltage and directional signals voltage are separate from each other.
Each has their own fuse.
Tail lights come on when head light switch is pulled out.
The stop light voltage gets routed through the Directional signal switch.
The tail light voltage does not.
1954 Truck Directional Signals wiringThe stop light voltage and directional signal lights flasher module voltage share the same rear routed wires,
(But never at the same time) depending on the location of the Directional signals switch assembly lever.
If you have the lever in the up or down position and then push on the brake you will only have
one brake light on and one directional signal light on at a time.
Pushing on the brake with the lever in the center position, applies stop light voltage to both the rear
left lamp (ribbed) and rear right lamp (purple) wires.
In effect you just turned directional wires into stop light wires.
Picture of fuse (pink) on rear of instrument gauge cluster. (courtesy tclederman)
The fuse supplies power to the flasher module and directional lamps.
If the fuses are good.
You may have a bad
Directional signals switch Or cut or bad connections in the stop light & tail light wires.
Picture of a Tail & Stop fuse holder. it is mounted on firewall next to drivers side hood hinge.