Please read the below post from bhen the water drains around the door seals
blah blah on how to properly post if you want proper help in the future,
As you didnt state anything about your car, I will assume that you have an
S-series, which is known for this problem, which I again assume is that when
water drains around your door seals (working as designed) the water collects
under the snap-on door sill plate.
All S-series do this, it is a piss-poor design, but is not an imediate rust
threat if you keep that area dry. A pain I know, but I run a thin towel or
chamois under that plastic rocker panel cover to absorb any excess water. I
have seen many s-series with rust in that area because it was not kept dry.
Also, on SC2's keep the 3rd door latch dry, it will also rust.
On the 1st gen S-series, the snap on plate covers a gaping hole, harder to
work with. On the 2nd gens, up to '02 models, the plate snaps into three
small slots in the rocker panel. What sux is that this is also covering the
weld seams on the rocker, so beat to keep it dry. As the cover isnt solid,
no luck on permanently sealing it up for me, any ideas anyone?