I had to find the cause of water ingress on my cousins gtv and after
much stripping out of the interior to trace the drain pipes from the
sunroof found the cause. The drain pipes run from the sunroof along the
sides of the roof and down the rear pillars, they then go into the boot
space behind the side trims and exit through holes in the floor, one
either side. The ends of these pipes can be seen just by laying under
the rear of the car. In the case of my cousins car the nearside pipe
was excessive in length and at some point someone had pushed it from
the exiting end back through the hole in the floor to match the offside
one for length. This caused it to kink behind the side trim in the boot
and restrict flow of water so that once the pipe was full the water
would build up in the sunroof recess and leak into the cabin. The fix
was a lot easier than the strippping of the interior, just pull the
pipe back through so there was no kink and chop off the excess to match
the other side.