If you want the single barn door light to come ON when you apply the brakes. You have to run a separate wire from the brake light switch to the barn door light.
When pulling out the headlight switch, do you have a tail light in the barn door light ?
Normally they wired the panel trucks, (that have the rear left and right directional lights) to just have stoplights on the corners of the vehicle. The barn door light was just a tail light.
You are essentially switching between two sources, the flasher module and the stoplight switch. If you connect the barn door light to one of the rear signal/stoplight wires, it would work as a brake light until you used the turn signal switch. Then the barn door brake light would turn into a directional light. The problem is the brake light switch goes through the turn signal switch. Hence the need for a separate wire from the brake light switch to the barn door light.
Note: When running a separate wire from the brake light switch to the barn door light, and the turn signal switch is IN hazard mode, the barn door light will not be connected to the flasher circuit, thus will not be flashing. Though will come ON when applying the brakes.
