On some of the older Mopars, there are plastic or rubber caps in the holes in the lower INSIDE OR UNDERSIDE body panels (above the rocker panels). I removed mine since a couple of areas were full of water. I guess this is the thing to do, but don't understand why this wasn't done at the factory, to prevent rust in the rocker panels.

 

i put a about a half inch of oil into my doors to stop rust, and drain it out to change it every other winter. oddly enough i dont have a plug, but i drilled a hole and tapped it so i can use a screw