Shelf under sloped rear window of a car
What is a good word or phrase for the shelf under the sloped rear window, behind the back seats of a sedan-style vehicle?
It's a parcel shelf:
(automobiles) a shelf (which is usually removable) inside a car, located behind the back seats, and used to store small items. There were a pair of toy dogs on the car's parcel shelf. (-- Collins)
Edit - although the definition says 'removable', in my memory it was always used for fixed shelves prior to the widespread appearance of the hatchback (British English).
It is called rear deck, deck, or more colloquially, rear/back dash in the US.
deck: the horizontal surface at the rear of the car, which usually serves as the trunk lid.
Glossary of Automotive Design
The flat area extending from the top of the backseat to the rear window is called "the deck" or "the rear deck" in American English.
The commercial sites I looked at for alternatives confirm that Myridon's "rear deck" is used as well as "rear dash". Here's a link to a picture of covers that are being sold to cover the "rear deck/dash". I think "rear deck" is the preferred term for those who actually work with cars. Some of the sites use "rear dash" in their titles so that people can find them with a search engine.
Word Reference
Once he was sure the boy would stay, he placed the crumpled papers and the school cap on the rear deck and closed the door. (The Taximan's God)
Because the man was wearing a suit coat and unfashionable glasses, Rollins at first had guessed banking. Then he spotted an umbrella on the rear dash, and he reconsidered. Insurance? (The Dark House)
She spotted her Indians baseball cap on the back dash. (Please Reply)
I've always called it 'the rear shelf'. This is supported by a Google search.
Example
"Can you get me my sunglasses from the rear shelf?"
The existing answers address the actual name of the shelf. I know exactly what you're referring to, and I'm sure auto manufacturers have a name for it, but I've never heard it referred to by name since there is rarely any reason to talk about the shelf itself. More often, one would refer to objects placed on the shelf or the act of placing them there. In that case, I've always heard it referred to with the idiom in the back window. (US, various regions.)