[binty]

Sorry I wasn’t clear but Harley Dave is correct.
The upper door is two pieces of plastic bonded together inner and outer.
Over time the bonding can start to have gaps in it at the top edge between the hinge and corner.
On mine water was getting in this gap, then running invisibly in the box section at the side of the window. Then down inside the inner trim panel on the rear and eventually becoming visible when it dropped out of the Bottom of the trim panel on to the lower boot lid.
It was easily fixed by spraying a few coats of waxoyle onto the gap between the inner and outer panels at the top, letting it dry overnight and smoothing it in. The gap will have been small but enough to allow the water in.
Completely cured it (once I found it).
Hope this helps.