Satellite radio company faces $1 billion debt, hurt by lagging car sales Barely three months after the long-delayed merger of satellite radio companies Sirius and XM, the newly combined Sirius XM Radio is struggling to stay afloat. The company has just another three months to start paying down more than $1 billion in debt that's maturing in 2009 at a time when credit markets are freezing up. It remains heavily dependent on automobile sales for new subscriber additions just as U.S. car sales are tanking. And its stock price is in a yearlong free-fall that has sparked an investor lawsuit....