Ford Motor Co. plans to build a new $1.6 billion auto assembly plant in Mexico, creating about 2,800 jobs and shifting small-car production from the U.S. at a time when moving jobs south of the border has become a major issue in the U.S. presidential campaign. The company announced the plant in the San Luis Potosi state Tuesday without saying specifically what cars it will build there. But the United Auto Workers union has said Ford plans to shif…