Ford Motor Co. and Mazda Motor Co. will lay off about 945 employees, or 31 percent of the work force, at their jointly owned plant because of slow sales of the Mercury Cougar and Mazda 626 coupes. The plant in Flat Rock, Mich., known as AutoAlliance International will eliminate its second shift in July, said Ford spokeswoman Cheryl Eberwein. About 875 hourly workers and 70 salaried workers would be laid off, she said, but those numbers could change before July. The plant has 2,843 hourly and salaried workers.