Hello & Welcome to JA,
The power to this light bar comes from a stop lamp fuse under the rear seat. If the other stop lamps work I would suspect an open circuit at C421 leg of the circuit.
Center high mounted stop lamp (CHMSL) harness to body harness, center rear of the rear compartment on the rear compartment lid / C421.
Rear compartment lid harness, rear compartment approximately 28 cm (11 in) from the high mounted stop lamps.
Check for voltage at these places when brakes are applied.
The center high mounted stop lamp (CHMSL) is located along the rear compartment lid. The CHMSL operates simultaneously with the rear stop lamps and provides increased visibility of vehicle braking.