Welcome aboard.
Firstly, where is it getting wet?
If it's one side or the other then it's most likely to be the screws that mount the rear light trims letting water past them.
If it's in the middle and round the battery tray then the main culprit is the third brake light: either cracked or the seal has failed.
Also consider whether it may be building up in the wells in front of the rear wheels and spilling over.
Then you are looking at moving the roof motor into the boot before it dies (which tbh, you should be thinking of anyway)