Hi, I have the exact same issue.
I have sealed the light unit and fixings but it seems to leak from above it
As you say it looks like it's from the roof trim but I removed mine, just held in with spring clips, and found no holes or anywhere water could get in.
My next job is to put an endscope inside the bodywork at the rear.
Please let me know if you have had any luck.
I will report back my findings.
MarkV