I imagine that most new cars before the current crisis were purchased by using a home equity loan, as this is the best way to manage borrowing $12,000 to $80,000. The interest is not deductible unless it is on your home. Of course, this is against the spirit of the home interest deduction.
If you are in the 25% bracket, a $30,000 loan at 8% would be $2400 in interest, and the deduction would be $600 per year for maybe 5 or 6 years.
If people only bought what they could afford, meaning buy for cash, then the average car would not be junked after 8 years, and more people would be driving Cobalts, Fits, Corollas, Rios and Accents.