I just completed my wiring on my 54 GMC and the brake light is wired through the turn signal like this schematic. It is a hot wire for the brake lights and not a ground. This allows you to run use the turn signal wires as brake wires and you do not have to run a separate or dedicated brake wire. The wiring harness I used said that it was up to a 54, but the schematic and the way it was wired was for a 53 and earlier with a dedicated brake wire. The turn signal bulb was off the flasher, one bulb. The switch then controls the light for brake or turn signal so you loose the brake light on the bulb with you have the turn signal on.
