The downside of owning a global brand is the awful possibility of screwing it up. Ford holds its collective breath whenever it has to roll out a new F-150 pickup. Early reactions to the Cayenne crossover must have had Porsche managers back-dating their résumés. And at Jeep, straying too far from the original mil-spec formula is guaranteed to set the Timberland-and-Carharrts brigade to howling.