There is a rubber weather strip that runs around the door near the seams.

When the door is open you should see a dirty line where that weather strip contacts the car body.

You should be able to follow it from the A pillar, down and right the way around the the B pillar.

If you dont have that dirty line it is either because you recently washed it off or the weather strip has shrunk a bit over the years and not making a good contact when the door is shut which can let rain and road spray get passed the weather strip and inside the car.

I do two things:

1.

I sometimes wash the shuts and the door seams and weather strips which cleans them and puts a wax layer on them to prevent rust.

(Dont use too much water as you dont want it to end up in the footwell, and dry it before closing the doors)
2.

I use rubber rejuvenator on the door weather strips and the rubbers in the door shuts.
