It looks to be the back of the bed has either a bad seal where rust got in or rust. It has a cover so it won’t be the cab. I was worried more about the vhs tapes and glad to see the titles had work related titles on them.
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Sorry, re-read what you asked. No, the drip rails are not rusty. Mostly moss and dirt. Has a terrible paint on it, but the truck is pretty rust free with mostly mild surface rust that I can see (so far).