Water ingress in boot

[Woodsy]

Hi everyone. 
For 2 1/2 years I've had water entering into the car through the interior trim of the tailgate. Not sure how this is getting in but it's evident after it has rained heavily. 
The water seems to gather inside the tailgate structure and then enters the car through both of the two pop out plastic grey covers behind the tailgate lights. It's also come through the tailgate locking mechanism. The three drain holes are clear.
I notice it when the tailgate is opened fully and then the water drips out at varying rates.
It has now been into VX 15 times and had rear lights sealed, rear window re sealed etc etc and finally a brand new tailgate but the problem is unbelievably still there. Car is now out of warranty but its going in again next week - but I don't hold out much hope to be honest! 
Has anyone come across this problem before or know how to rectify it once and for all? It's driving me NUTS after all this time.
I have pictures but cannot figure out how to post them.