MEXICO CITY—Freezing weather that interrupted gas supplies in the southern United States and Mexico was wreaking havoc on Thursday with car manufacturing plants on both sides of the border, with Ford saying several of its assembly plants have been shut. The cold snap has overwhelmed Texas’s power grid, while natural gas supplies to Mexico from Texas were interrupted, leaving millions without power in Mexico’s industrial northern states earlier in the week. Mexico generates most of its power from natural gas, largely imported from the United States. The two countries also require intricate supply chains to be functional to supply auto and other industrial operations on both sides of the border. Ford Motor Co. on Thursday said adverse weather had led to the temporary closure of plants in Kansas City, Flat Rock and Ohio, as well as a plant in Hermosillo in the northern Mexican border state of Sonora. Late on …