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I pulled the sill plates off, Put a spray tube like from a can of carb cleaner on a can of rustoleum, & unloaded a couple cans in each side. Dry them out first with an air hose & blow gun if You can.
there are 2-3 rubber plugs about 1-1/2 inch diameter on the vertical back side of the rocker panel. the panel is spot welded to to the vertical plate 4 inches back, under the door. The spot welds are not sealed. It should leak easliy thru in this area. Pull one of the rubber plugs and look inside. YOu can also look thru around the running board mount area. Specifically the front one. There is a 2 x 4 inch cutout in this area on the vertical wall.
I didn't say spray Carb Cleaner in there. Rustoleum with a spray tube on it "LIKE" one from a can of carb cleaner to get in the holes. Once You dry it out, It's going to get wet again. Stopped Mine from rusting out.