Central Brake Light (Leds)

[Cymro]

The "bulb failure" warning light has come on. So I checked all lights and combinations of lights - and the only light which is not illuminating as it should is the central LED brake light in the top of the tailgate. I suspect it may be a wiring problem rather than a failure. But how do I get at it? With the tailgate open, there's a pice of plastic trim along the inside top of the tailgate, under which sit the LEDs.

Can anyone advise how to remove that trim, please?

Or - if anyone has had this problem - can you advise how you sorted it?

Thanks