"the Big 3 enjoy political protection from such competition"
Right, as a result of the Chicken Tax, an extra import duty of 25% that was added to light trucks in 1963. The tax was added in response to France imposing a stiff duty on American poultry products. To avoid the 25% tax, auto makers can build the vehicles here, or import them as kits, and assemble them here.
The Chicken Tax applies to USA manufacturers if they try to import a vehicle they build in another country. For the Transit Connect, which is manufactured in Turkey, Ford does an end run around the tax. Ford imports all Transit Connects as SUVs by adding glass side windows and a second row of seats. The import tax on SUVs is 2.5%. Once in USA, Ford replaces the windows with steel panels, and removes the seats, converting the SUV back into a van.
A similar strategy was employed by Subaru. In the 1980s, Subaru made a small pickup. By adding seats and seatbelts to the truck bed, Subaru was able to import the truck as a passenger car called the Brat.
Crazy, no?