Ain't had the problem, but . . . I'd take a voltmeter and see if + 12 volts DC appears on the "hot" side of the switch with ignition key on.
Next, I might take an ohm-meter and see if the ground side of the switch actually connexts with ground.
Next, I'd probably disconnect the hot side of the switch, connect an ohm-meter to both sides of the switch, and see if continuity exists when brake pressure is applied, and if continuity is broken when brake pressure is released.
With all these tests "positive," I'd reconnect the brake switch and expect voltage at the brake light when ignition switch is on and brake energized.
Still no brake light? Then, I might check the brake light wiring and brake light bulb filament . . .