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Try Mothers "Back to Black". It seems to work on the cladding and would probably work on the running board on my wife's 04 Expi. I use it on the black plastic on my 02 Toyota Tacoma p/w It has black running boards, black fender flares and the front facia under the bumper, and the black on top of the rear bumper. I also used it on the plactic cladding on my wife's 2000 Exploder and it workered. It's more of a polish/wax type that doesn't build up and turn white and is not slippery like Armorall
I bought a 79 GMC pickup with rubber floor mats an vinyl seat. I used armorall on it and almost killed my when I got in, slid all the way to the passenger door. If it was not for seat belt I would have not been able to drive it. New seat cover next day. and carpet on the floor.
The black plastic/rubber piece on top of the rear bumper and the running boards on my 03 EB are fading - I used Zaino's "perfect tire gloss" product on the rear bumper plastic a couple days ago - it stil looks black, and it has an even, matte finish - not glossy as the name would suggest. I want to see how it holds up before i do the running boards. I've tried the black chrome product, it's too glossy for my taste, and in my experience the armor-all starts to get splotchy within hours.
Be very careful if you coat the running boards with any kind of "protectant" as they become wicked slippery and you could easily go for a loop when you stand on them getting in/out. Only my extreme athletic ability saved me from doing so when I once (and only once) did mine.