Wrenchbender is correct.
The directional signals were an add on. Either installed by a dealer or the owner. The kit was independent of any of the lighting on the vehicle. The kit had 2 lights that mounted in the rear (side of the frame or bed depending on truck type) and 2 lights that mounted on the tops of the front fenders.
The wiring was simple, 1 wire for the right and one for the left.
The brake light(s)were not involved.