Originally Posted by Alphamale

Just behind the gutter corner trim just at the corner of the screen where the gutter meets the 'A' pillar inner and outer skins, roof and inner top of the side panels all the panels are meant to be sealed against water ingress, but in many cases this was missed of badly executed.

Remove the 'A' pillar trim and corner section of the gutter. examine the panels where they met in the corner of the screen and you will likely find a hole or loose filler. Clean away all the old filler you can get out and degrease the area ready for fresh sealer. I would not recommend a silicon based filler as they do not adhere for long. When I did mine I got a tube of car body sealer from my motor factor shop.

It is likely that you will also find rust in the area that you must clean and treat properly to affect a good repair. You will also find that water will have traveled down inside the 'A' pillar and could have started to rot the metal in the scuttle below the screen just where the 'A' pillar passes the fender and bonnet hinge also check for water ingress behind the fuse box and connection by passenger kick panel.

Looks like you'll need a new head lining too.