Once there is a self-driving (electric) car that you can order from your phone and that will — cheaply! — take you from point A to point B, the need in owning a car will (mostly) disappear.
The people would stop being car buyers. Instead, robo-taxi companies will buy most of the cars. For these, there would be no need in brands or marketing; some of them would just assemble and service their own fleets from readily-available parts, similar to how Amazon/Google/Microsoft self-assemble their own cloud servers (instead of buying them from Dell/HP).
Some people would still want to own their cars, like today some people own horses. They would be a negligible minority. There, it's likely that at least some of the manufacturers would be those who still don't exist today; they would disregard the no-longer-relevant mechanics (internal combustion engine, gearbox, etc.) and focus on overall end-user experience while utilizing readily-available parts.