Buying (financing) a new car, and then having it repossessed at half the purchased value (still 50%) debt to pay
If people have cars repossessed en mass, the second hand car market will be affected, prices for used cars go down (because the supply goes up). This makes repossessions not cover the whole loan, see above point
Finally, as more and more Americans no longer have access to cars, they also lose access to… A lot of society (work, education, healthcare). If this affects a large enough percent of the population, there will be macro effects.