Yours must be pre-LCI? Could be your tailgate led strip mind?
On the LCI tourer, and there is a known water ingress issue with the LED tail light clusters.
Funny thing is the first thing you get is bulb warnings, but all the LEDs seem ok, but look carefully and you may notice just 1 LED element not working.
I had just one fail in the indicator LED matrix and this triggered the bulb fail.
But because LEDs 'don't fail sir' you can't buy the LED units, you have to buy the whole damn light cluster at £175!!