The import of German cars isn't a binary situation. Its not going to be situation where imports are banned. At worst the price will increase due to import tariffs. That will impact them but only by leading to a decrease in sales, the way some people are talking you'd think they'll never sell a car here again.
And of course that assumes that the import tariffs aren't offset by changes in the exchange rate and / or there aren't changes to prices that can be made by the manufacturers. German cars costs way more here than they do in other EU countries, so they have room to play with if we impose tariffs.
You also have to consider we're not dealing with one industry in isolation. If we go hard on car imports we'll get hit somewhere in return, most likely the service sector.
At the end of the day the EU aren't going to allow us to be in a situation where we're better off leaving than staying, if they did that the whole thing would collapse. Best we can hope for is a Norway type agreement where we keep paying in to get access to the single market and accept freedom of movement. Essentially we'd be in the same position as now expect we won't get a say in anything.