Well, to be pendatic, they do have parts called boots. They are the rubber protectors that go over electrical connectors, cv joints, tie rods, etc. But as far as I know, there are no bonnets.

You can have one added after the fact, though http://philapark.org/wp-content/uploads/P.4.CarBoot.jpg

Junk in the boot, just don't have the same ring to it

Oh, sorry what do they/you call them? (The storage above the rear axil at the rear for the car)