Lift the tailgate, in either side of the shut there is a drain. A couple of things happen here, either the drains get blocked, which is common, and the water can't escape and runs over the top of the seal. Or, as happened to mine, doh ! the drain pipes which run inside the rear trim panels and out through the floorpans come adrift from the drains and the water runs directly into the car. In my case it was because we had been working in that area but the normal reason is that someone has tried unblocking these drains with a bit of wire which pierces the pipe or pushes it off or they have blasted it with excess air pressure.
Look under the rear quarter on both sides of the car and you should see a pipe protruding.
Remove the trim panels, a bit fiddly and time consuming but worth the effort and replace the pipe with a suitable length of gardn hose, much stronger and will drain better.
This is the normal cause. Other possibilities may include poor fitment of the tailgate if it has been removed, poor seals around the rear lights [doubtful though] or if you have the moonroof option, a poor seal there.
My guess is the first one.
