Noe, I don`t have this feature on my car, but I would imagine your high-level brake light contains LED`s rather than bulbs.......if this is the case, than it will be sealed-for-life & non-serviceable!.
Do both the normal (bulb-containing) brake lights illuminate when the brake pedal is depressed?. If not, this could be the reason the current is not reaching the 3rd. unit.........replace the bad bulb/s.
If they both work normally, then any local auto-electrician, not just the dealership, can sort you out cheaply.
Cheers,
Nick.