New York Times: THE “cash for clunkers” program will be back in full swing this weekend after President Obama signed a program extension on Friday. The $2 billion infusion should help dealers sell thousands of cars to customers who can reap a bounty of $3,500 to $4,500 for trading in an old vehicle. The short-term effect will be obvious, but the program’s longer-term value – either as a method of stimulating the economy or saving energy – is less clear. Economists say that most buyers simply moved up …