General Motors Corp. is going to bat for the University of Michigan, which is the target of two lawsuits over its use of affirmative action in determining admissions. In an unusual move, GM, which says it employs a large number of U-M business and engineering school grads, has filed a brief supporting the U-M's tactics. In the brief, which appears to have Vice Chairman Harry J. Pearce's personal stamp, the automaker contends an ethnically diverse student body prepares graduates to function in global marketplaces. Failure to attract minorities to the school would "deprive business of the well-trained minority candidates who are essential to our nation's economic success," GM says.