DETROIT——Ford Motor Co. last month announced that it was canceling plans to build a $1.6-billion small-car assembly plant in San Luis Potosi, Mexico. But, the automaker is still going ahead with plans to open two other new plants in the country: a $1.1-billion engine plant in Chihuahua, and a $1.2-billion transmission facility in Guanajuato. Both are planned to become operational later this year, supplying engines and transmissions for the US, South America, Europe and Asia.