Water leak under door weather strip?

I've had this leak for awhile now. When it rains and the truck is parked in my sloped driveway with the front end pointing uphill, it gets wet around the base of the seatbelt. Only on the drivers side. I will sometimes pack a rag in that location or even better, turn the truck around if rain is expected. Front facing downhill, no water gets in.

I have noticed that water seems to build up under one part of the weather strip (photos below), only on the drivers side, which has me suspecting that it somehow manages to get under the weather strip and then up over the pinch weld and into the cab. Has anyone else experienced this? I thought I might have read a post or thread years ago describing a similar issue but have no clue on how to find it if I did.

Also, yeah yeah, I know I should have delt with it long ago before it got this rusty. It will get handled. My only concern at the moment is understanding how it's getting in, if others have experienced the same, and if yes, how it was remedied.



(The rust at the bottom is mostly just superficial, rubs off)