In the fall 50/50 diesel fuel and motor oil in a bug sprayer. Spray inside doors, inner fender wells, inside tailgate inside rockers and everywhere you can think water can get on the under side. Let it sit and drip for a few hours. Then a day or two later wash the out side of the truck. The truck will not smell like diesel. We do it to my dads 07 dodge 2500 4x4 no rust and yes I am in Michigan. Or you could do fluid film and do the same thing. But you are going to have to do it every fall