(Spot what's missing!)
After consulting the knowledgeable regarding the replacement of the window mechanisms, I found out some more info as to the presence of that plastic on the left door.
It turns out that car factories did fit them to cars (Fićas and otherwise), just not in the way that the restorer did.
The correct way is to attach the nylon along the top edges only, allowing air to circulate while blocking the ingress of any water on the wrong side of the door cards. Then it hangs freely on the inside of the bottom of the doors, allowing any water to empty into the drains.
The factory window seals (only referring to windows that roll down) inherently let in a little water: it's not like the tight seals that modern cars enjoy. As a result, the nylon membrane serves as a second line of defense when the wet stuff begins to fall.
The restorer decided it was a smart decision to glue an ill-fitted piece of nylon almost all around – which was preventing air circulation – and then only on the left door, leaving the right one exposed.
But hey, it's on there, right?
Wrong! Very wrong.
He also "forgot" to paint the inside of the right door completely (as you can see), and there's some padding missing beneath the long plate that ties the door handle and the lock mechanism.
So, at the very least, I'll have to replace the window mechanisms, clean out the drains inside the doors, cut and properly mount the new nylon (No glue! It doesn't play well with water.), and put the door cards back on.
Oh, and find and mount the missing passenger's side door stay and then align that door.
The doors have turned into a pretty comprehensive project in their own right.
And then there's the electrics. But I'll save that for next time.