Sergio Marchionne enters the final stretch of his career at Fiat Chrysler Automobiles NV with a daunting mission: prove wrong the doubters who question how he can make the carmaker’s $5.29 billion in debt disappear by the time he steps down in two years. His strategy update will be a closely watched presentation next week at the Detroit motor show, particularly as Fiat Chrysler won’t be unveiling new models at the annual event.