2 of 2 added 180 characters in body

The fact that the light bar lights up means that it works. If the fault is consistent, I'd be more tempted to look for the fault at the brake pedal - where the switch for the light is.

As your existing brake lights work correctly, next step is to see whether they use the same switch, as per @Brian's comment. It may be that the LED bar has been wired in completely independently of the original lights.