Cheers Alex if you hadn't replaced your rear bumper & mentioned that you were missing a grommet on the O/S/R behind the bumper i'd have never checked mine
sure enough Nissan must have penny pinched here aswell as I was missing the same grommet
popped to the breakers & pulled one off a MK2 T2 & got it fitted & dried out the slight amount of water that had built up in the rear 1/4
so anyone with a late model T2 i'd check to see if you have the grommet fitted as rain water easily gets in if the grommet is missing