Just a thought as I'm currently chasing the source of a very leaky boot as well - are either of your rear lights cracked? I've just replaced one of mine that had a relatively minor crack and once it was off the car I filled it up from the inside using a tap and it was surprising how rapid the drip through the crack was.
I've also replaced the rear gaskets with a set of these, so next time it rains I should know if the rear lights were the main source:
https://www.the75andztclub.co.uk/for...d.php?t=142690