You could start there. Call it an "inverse savings bond" where instead of the 12% you're paying on your car loan, the Treasury could buy your debt from the bank and you'd be assigned to essentially pay the coupon on a 10yr bond (1.88% today?). The banks wouldn't even have to take a haircut: they'd get their principal back early, and that would be a bummer for their ongoing profitability, but they'd be made whole.
Now what they'd do with all that money is another thing ...
/jordan