Just want to make sure it's not the sunroof drains blocked.
Having gone as far as you have, I'm sure you've ruled that out (assuming you have a sunroof).
Otherwise, you rarely hear of the rear windscreen seal going on any car. Best bet is a dealership or windscreen specialist to have it resealed. You will want to know how it's bonded in the first place though in order to seal it properly, so I would err with caution and go with the dealership.
I would check for corrosion too. Just because its Aluminium (spelling is correct
), doesn't mean it can't corrode.