1 brake light out....turn signal switch?

If you are CERTAIN the signals work normally, it is the switch. The TS switch indeed steers the brake light switch voltage depending on which signal is energized, AKA with no signal, the TS switch sends brake switch power to both left and right signal filaments. With the left TS selected, the switch continues to route brake power to the right filament, but interrupts brake power to the left, and instead, connects the flasher to the left filament.