DETROIT — Mitsubishi Motors Corp.'s departure from U.S. manufacturing more than two years ago left the central Illinois town of Normal, population 52,500, with an empty 2.6-million-square-foot auto assembly plant and more than 2,000 trained workers without jobs. As industrial sites, such brownfield factories are tough sells in an auto industry that is cautious about investing, leery of taking on new plants and often more eager to build modern greenfield factories than to take on someone else's castoff.