The car business is pretty good right now. After bottoming out during the financial crisis, sales have been climbing steadily. But all good things must end and the auto industry is brutally cyclical. What's worse, the down side of this cycle could be incredibly steep. Automakers are expanding production to take advantage of the strong demand. They could be left with vast over-capacity as demographic shifts away from the car culture -- and perhaps rising interests rates -- result in far fewer buyers. One leading executive believes only big companies will be able to compete in the global car market. Could we see a merger between Ford and Honda? How about Fiat and Volkswagen?