When writing about Nissan's Murano CrossCabriolet recently, I was reminded how difficult it is to create a new automotive concept -- and how harshly the market treats original ideas. The CrossCabrio is the ambitious -- or perhaps foolhardy -- attempt to create a crossover with a convertible top.

    Bright ideas are hard to pull off. For every Chrysler minivan or Ford Explorer four-door SUV, there are dozens of one-hit wonders and new concepts that failed to catch on. The Pontiac Aztek, an unholy combination of minivan and SUV, is the classic example, but there have been many others through the years.