Space in trunk by the fenders - trash

That's often part of the reason the quarters rust out at the bottom - dirt and crap get down there, retain moisture, and rust from the inside out. In mine, in addition to the dirt, I found old lug nuts. It was rusting through where the lug nuts were lodged.

Once you get it cleaned out it wouldn't hurt to apply some anti-rust paint or coating down in there to seal it up. There's sure to be some rust growing.