The rear bumper being plastic won't rust, ok you may get some items wet if you load/ unload the car but it makes a really good wash tray for wetting the sponge for the window cleaners at traffic lights
Yeah this problem occurs with my MW. I also park mine under a tree and the amount of tree debris that gets into that space is amazing. The area where the foam collects responds very nicely to some cockpit shine if it has become dull or scratched