Having water run down there isn't really the issue... it's the seam sealer they used - it's dried out, cracked and holds water./moisture behind itself, on the panel. The whole lot needs stripping off, rust removed and re-sealed with a modern urethane or modified polymer sealer.
The factory sealer is crap - My late disco 1 has never seen a salty road in its life, and the sealer was still causing issues (plus the hole it creates at the windscreen base). Mitsubisji had a similar problems in the 90's in japan and it rotted out roof panels, right in the middle, where the cross-braces were underneath.
I did add a bead of the same sealer, so any water from the scuttle ran directly off the box section, instead of running along it's length.
After stripping:
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