As far as keeping them out, it's best to take measure to keep them out of the truck entirely rather than just the pillars. What o mean is good sealing and when it's in storage having mouse repellents outside as well as mousetraps.
The rust is from all the acid in their droppings, or mess, as you called it. I always just vacuum, pressure wash, etc. As best I can. And use a mask when you're cleaning the mess.
I dont know of product to coat the area, can you spray rust converter in there?
