You can bring some cars back. Years ago, I was actually contacted by the local office of the ATF, to help a South African immigrant to the US, who had gotten some spectacularly bad advice regarding his personal shotgun and his late 1930s Morgan automobile. No one at ATF can actually import a gun, you have to be an FFL or an importer to do that. Someone at US Customs told him to bring both of them in the container with his household items, and both were promptly seized when the container was opened.
The gun I could help him with, the car I couldn't. The gun was less than thirty days from being destroyed by Customs when I sent the paperwork to Washington through the local ATF office, with their promise that it would be expedited. They actually did do that. The gun was cleared before the car, but I never did hear how the car ended up.
There exist areas of regulation that overlap between ATF and US Customs, and from what I've seen, both agencies are uncomfortable when they have to make a decision that involves those areas. One would think it would be cut and dried, but a CYA attitude often exists that makes headway difficult.
Best,
Ted
