Ordinarily not much, but go ahead and listen to the first five minutes of this video.
I wouldn’t think Those Evil Foreigners would respond to tariffs on foreign cars being imported to the U.S.A. by imposing tariffs on cars exported from the U.S.A., since the U.S.A. exports relatively few domestic-built cars. (Which company exports the most domestically-built cars by value? BMW!) I would expect they’d respond with tariffs on products the U.S.A. exports a whole lot of.