Consumers often purchase a new vehicle while still owing thousands of dollars on their old one, which they use as a trade-in NEW YORK -- A growing number of new car buyers are finding they owe more on their existing car loans than the vehicles are worth as trade-ins. The phenomenon, known as being "upside down" on a loan, is the result of a confluence of changes in the ways Americans buy and finance their vehicles. To begin with, the prices of new cars and trucks have been held down as manufacturers offer incentives and rebates to lure purchasers. As...