Unless sooo much water is getting in that it's running along the rear panel seam, but even then it would have to overflow the lip where the latch is. That said, it could be running down the loom between the rear lights, but it would be coming from higher up...Any evidence of rust at the bottom rear corners of the rear quarter glass? If yes, then expect a hole behind the black trim. Easy enough to access: remove the inner trim from the C and D pillars and undo the twenty thousand 10mm nuts. Window then comes out and you can access the screws that hold the trim in place. Should be possible to effect a decent repair without it showing past the trim... But whatever you do, don't fill the trim with sealant.
The body vents are into the wheel well, but unless the car was reversed into a pond and left there it should drain out through them. Incidentally those vents are designed to open towards the low pressure at the back of the car as it moves forwards and hang closed when stopped. Ignore all of that, it's an Astra, not the Omega
Even my mechanic didn't believe you could change just the valve until I showed him the pictures from this forum