There is no issue of reservation of powers here. States are responsible for registration and titling of vehicles in their jurisdiction, the purpose of which is primarily to protect property ownership and facilitate taxation.
The federal government regulates vehicle safety and emissions, because cars are such mobile things, and it would make no sense to have different standards in each state.
On that basis, every vehicle under 25 years old permanantly imported into the US must comply with federal safety and emissions standards, and have stickers attached to the body certifying compliance, either from the manufacturer or a registered importer. That's federal law and applies in all states.
Think of it as the automotive equivalent of an illegal alien. Just because someone makes it across the border illegally and then manages to get themselves a state issued ID, doesn't mean they can't be deported later on if they get caught.