Most of the time, asking to borrow someone's car will elicit a really long string of excuses. But unlike letting your buddy use your car, which usually gets you nothing more than an empty tank, GM has found a way to make car sharing beneficial for everyone involved. If you're one of the 6 million people who have a GM vehicle that's connected to OnStar, you're in luck: according to an OnStar employee, you can make up to $1,000 a month just by letting other people rent out your car when you're...