Perhaps in his native tongue, the term assigned to the glove box is similar to "pocket." Sometimes people in the US chuckle at the British term "bonnet" for what we insist is the "hood." But "hood" and "bonnet" are not really that far apart.
To the best of my knowledge, "car pocket" is not a "set phrase" in English.
The car pocket is the, well, pocket set in the inside of the doors, where you can keep atlases, etc. I'm not sure that all cars have them, but our last couple of cars have. (But, as you say, Roald Dahl may not know the phrase and has used it as foreign-speak for the glove compartment.)
Yes, the pocket on the back of the seat is possible, too-- a good place for the registration papers, since a car thief might be hesitant to rummage through all those dirty kleenexes and sticky popsicle sticks.
I grew up In South Carolina and I suppose my native tongue is English. What people call the glove compartment or glove box was (and still is) called the car pocket. I read on line a few years ago that they are known by the same term in ome places in Texas.
Another local term for the area where passengers rest their feet is "the foot" as in the following sentence:
I also grew up in South Carolina, and my father always referred to the front compartment as the "car pocket". My husband, who is from Indiana, makes fun of me when I say it.