DC has the highest % of foreign cars being sold. Idle speculation: I wonder if some foreign embassies mandate buying their national car brands, and if they sell their cars quite often to keep their fleet shiny new...
Buying a car every month or two and reselling it to auto dealers could provide a reasonable profit.
A few years ago, tariffs accounted for 25% of a cars price for SUV's and Light-Duty Trucks from the EU. I'm not sure if it's changed since the new administration has started.
https://www.state.gov/about-us-diplomatic-importation-progra...
DC is stupid wealthy.
And we all know wealthy suburbanites love their Toyotas and Hondas (and respective luxury brands) and wouldn't be caught dead driving an Impala let alone a <clutches pearls> Cadillac. So that's what the market gets saturated with.
Foreign cars that are not produced under approved brands from these countries would be classed as grey market and are subject to import laws, that would likely make your proposed mandate impractical.
Also the number of foreign diplomats in DC's ~690,000 population is unlikely to skew the numbers.