I suggest that you study this wiring diagram. Based on what I can best determine turn signals were a dealer installed accessory for trucks in 1954. This included adding a second dual filament taillight to the right rear of the bed.
Your latest picture only shows 3 wires in the harness from the steering column. There should be another 3 wire harness going up to the turn signal switch. It has white, pink, and purple wires.
1954 Truck Turn SignalsI am completely confused by the double dual filament tail lights. Until you can trace all the wires to all the filaments I doubt if I can offer much more guidance.