Hey, My led brake light only light up 3 LEDs, I thought they had just failed so bought replacement LEDs from powerful UK but the new ones did the same. Any ideas what could be causing it?
I've checked that the connectors are corrosion free.

It's a single unit so if not all are lit then it's defective. I bought a new one, not from there and it worked just fine

I had similar with one of the LED brake lights on my L322, it seems that the LED's are wired in multiple parallel/series circuits so if you have low voltage (caused by a corroded connector, or more likely on the liftgate, a fractured and corroded wire in the flexing part of the harness) you only get some of the LED's illuminating as the forward voltage on the remaining LED's in series isn't high enough to illuminate them but they'll still electrically complete the circuit.