As Bloomberg explains, for borrowers who do trade in their underwater cars, lenders are essentially giving them the money to pay down their loan. The dealer sells the used car, and whatever balance remains on the old loan is folded into the new loan. The borrower might get a longer-term loan than he or she had before to help keep monthly payments manageable. That means that loan balances are getting bigger relative to the value of the new car, and the debt will be paid off slower.
Daisy Chain In Autoland—-How The Debt Just Keeps Growing