Severe Leak - Is this common?

[UKDavo]

Hey Guys!,

So I've got this issue with a leak in my GTI, when it rains I usually find my passenger rear seat is wet (like completely saturated).

I never see the actual leak, only just the wet seat.

So I got in my car a few days ago and went to drive off, I kept an eye on the seat and saw this HUGE waterfall come cascading down from sill between the door and roof and completely soaked the seat again.

Is this a common issue? did some google searching and I've found people with similar issues but never in the rear.

Any ideas what it could be? I think I might end up having a mould problem on my hands if I don't get it sorted soon.

Cheers guys,

Davo.