Pull up the floor carpet and see if you see anything there. If there are any drain holes that drain into the rocker cavity, pour a little Ospho rust arrestor into it. Ospho will react with the rust and turn it into a hard, black substance and stop the rust from spreading.

I've had a few encounters with rust in the most extreme cases. Have a 92 Metro convertible with the rockers and fenders rusted straight through. Holes in the floor the size of basketballs. Can't figure out how it hasn't folded up like a taco yet with the lack of frame support. I'm hoping to get the holes welded shut soon.

Also, check your drain holes and oil up the rocker surfaces. Maybe get some mud flaps?