Is this a rhetorical question? We don't call any part of a car anything to do with hats or racks that I know of. (Except for a ski rack or luggage rack on the roof.) A hat rack in other contexts is either a tall post with hooks around the top, or maybe an open wire shelf, possibly with hooks underneath. (Oddly, LEO has "hatrack" only as one word, and that marked as [aviat.], which leaves me blank.)
For the part of a traditional car (sedan) I *think* you're talking about, the phrase that comes to mind is "shelf behind the back seat," but it's usually fixed, not openable. For a hatchback, wagon, van, etc. with an adjustable rear seat and/or no trunk, "cargo shelf" might be it, but there are probably other options too, like "storage-compartment cover" or something.
I didn't find any images of a Brillenablage in a car. A glasses rack would hold many pairs of glasses, e.g. in a store. A glasses case (Brillenetui) is usually portable, kept in a purse or pocket. I suppose a car might have a glasses holder, analogous to a cup holder or garage-door-opener holder (possibly clip-on). My car has a small flat space down in front of the gearshift where I keep my sunglasses (and coins and pencils). I don't call it anything. (-: