I'm getting water pooling in the lower area behind the right rear light after heavy rain. I can't seem to find the source.

I'm only getting water in on the right rear, left rear is completely dry. I'm not getting much in the boot but there is a trickle coming from the right rear area.

So far I've:

• replaced the rear tailgate seal (including bending the flange outwards)
• removed the rear light, checked seal and refitted. The seal seems to only come as part of the light so I couldn't replace that.
• checked sunroof drain - it's clear.

Does anyone have any clues what to check next?