Just had this
problem on my car.
I could see the water p*ssing out near the interior boot light whilst getting someone else to squirt the washers.
It was easy enough to fix thanks to this thread
Take the two screws out near the interior light and the one behind the rear seat then the trim just pulls off easily.
The interior light comes in handy as a torch so you can see what you're doing
Here's some pics to show the trim removed and the culprit: