Dealing with frame rot

I had the same rot in my Duster, it was cause by old wiper piviot seals, which leaked and filled the floor with water.
When the floor rotted out, water went in the frame rail.

Find the cause of the rot.

Mine was worse than yours, but I patched it with a cap, after cleaning everything up.
P.S. the caps do not fit worth a crap, be prepared to bend it to fit.

People who do not live in the rust belt, are scared of a little rust, it is just expected here.
That frame rail from the pic, does not look that bad, but inspect the full rail, tap on it with a hammer.