Ford is an international conglomerate now. We already have some tariffs on foreign automobile production, and the results have been Mercedes in Carolina, Honda in Ohio, Mitsubishi in Indiana, Toyota in California etc. (list may not be current but you get the idea). So tariffs do not have a completely negative impact.
An utterly punitive tariff against Mexico (which along with Canada now enjoys breaks due to trade agreements) would mean that Ford would convert their plants to 100% non-U.S. shipping, possibly as a spinoff or subsidiary. Trump knows that; he’s a high-level businessman. If Ford shutters or sells plants, then we can expect more Mexicans streaming across the border.
Ford is the last American centered car manufacturer that didn’t go bankrupt and let Obama stiff the stock holders. The rest of Trump’s plan will tend to raise the entry level price of having a car, that will filter all the way down to the used car market. OSHA and the EPA have done a heck of a job already with mandatory safety, emissions and CAFE standards, so that the cheapest car available is about $13K out the door, and a baseline family car is more like $25K.
The (potential)President of the USA wants a USA manufacturer to keep its plants in the USA. This is a morally correct and proper thing to do. It was what our founders did. It is what every other country in the world does that has a grain of sanity left.