DETROIT (Sept. 14)—General Motors Corp. and Ford Motor Co. were forced to stop production at various assembly plants as parts makers struggle to get just-in-time components across the U.S.-Canada borders. Commercial trucks were held up 12 to 18 hours at crossings between Michigan and Ontario, forcing Ford to shut down two assembly plants, one in Dearborn, Mich., which produces the Mustang, and another in Wayne, Mich., which turns out the Focus, a spokesman for the auto maker said. "We´re doing a daily assessment of the material supply just to see what kind of production we can do," he said. A GM spokesman agreed things have been tight. "We had a couple of plants where we had to send employees home." GM stopped first-shift production Wednesday at its Oshawa, Ontario, truck plant because of parts shortage. It had additional first-shift stoppages Thursday at its Janesville, Wis., large sport-utility vehicle factory and Pontiac, Mich., truck facility.