Ahem--the flat part at the back of a utility vehicle is known, logically, as "the back".
and what are the terms for the rear area of a station wagon, and for the shelf just behind the rear seats of a four-door coupe?

When the kids were little, we called the former part of our station wagon the "wayback". Later shortened this to "wabe" (pronounced with two syllables), in honor of Humpty Dumpty and Jabberwocky.