Check the scuttle drains before anything else.
Wouldn't that would make the whole car damp, rather than a specific spot? Checking the scuttle drains are clear
is normal maintenance.
For the stated problem, I would be taking off the door card and checking the top weather seals are good, the felt in the window tracks are OK and clean, the wiring and hinge lock mech grommets are in place, the door mirror has all its components and that the thin plastic vapour shield is still whole. I think it's highly likely that the shield is damaged, because most people who have been inside the door don't bother with it. A badly torn one can be replaced with any reasonably thick plastic sheet- damp proof membrane, rubble sack etc - stuck down with ordinary sealant if you don't have the
'proper' STUFF. Refit the door card, replacing all damaged clips and fasteners.