They may have changed how the magazine becomes illegal without changing the name of the offense i.e. a magazine with 10 rounds is an illegal magazine, a magazine with 7 rounds is a legal magazine.
US V Black was indeed what I was looking for, but while it strongly suggests the passenger isn't subject to individualized suspicion simply for being a passenger, that's about all it did.
Reading through
Justia's cliff notes on the fourth amendment and vehicular searches I think this guy is in trouble. IF the news article is accurate, I would assume they arrested the driver(as she was charged with at least three offenses in the story), which made a search of the vehicle legal (See the first page, right after Texas v Brown) and under
New York v. Belton - 453 U.S. 454 (1981) They can make a contemporaneous search of the passenger compartment, including closed containers. It would take quite a tap dance to argue a magazine in a firearm in the glove box isn't a closed container.
So I'm guessing unless the political winds are blowing quite hard against law enforcement in a post stop-and-frisk world, this search may well stand up. I hope the SAF, Tresmond, or the NRA start whispering in his lawyer's ear on the 2A part of his defense. He may need it.
Something along the lines of an exemption for ranges and competitions that aren't part of the core right as defined by SCOTUS making the core right more burdened than tertiary aspects of the right being ludicrous and unconstitutional maybe.
Edit to add, which doesn't mean there isn't a chance that Black will say/suggest that since the Officer got the firearm via a Terry Stop and he didn't have a reasonable reason to individualize the suspicion to conduct that Stop. But I wouldn't bet my freedom on that. Especially in NY which I don't believe is in the 4th Circuit.