Private investors have agreed to buy 25 percent of subprime auto lender Santander Consumer USA of Dallas for $1 billion. The lender, owned by Spain's Banco Santander, has more than $14.5 billion in contracts outstanding. More than 90 percent of volume is with new-car dealers, mostly for used cars.It is a preferred lender for Chrysler Group dealers. The investors are funds affiliated with Warburg Pincus, Kohlberg Kravis Roberts & Co. and Centerbridge Partners. The deal is expected to close by year end.