Mercedes Site List Narrowed: Speculation intensified that Mercedes Benz, the luxury German car maker shopping for its first assembly site in this country, was focusing on four Southeastern states as finalists. The Triangle Business Journal in Raleigh, N.C., and the Business Journal in Charlotte, N.C., said Mercedes scouts were looking at Mebane, N.C., and locations in Georgia, Tennessee and Alabama for the $500-million plant. The publications, quoting sources they said were familiar with the project, reported that the auto maker was believed to have put the Mebane site, located near Interstate 85, at the top of its list of North Carolina sites. Earlier this year, Mercedes announced plans to build a U.S. plant that will employ 1,500 people and assemble a sport utility vehicle. Production is scheduled to start by 1997.