CULLMAN, Ala. — Tuesday marked 25 years since the first Mercedes M-Class came off the assembly line at the company's plant in Vance, Alabama. It was the moment which ushered in the birth of Alabama's modern automotive industry. Today, the state has five automotive manufacturing plants and is poised to produce 1.3 million vehicles each year, the third highest total nationwide.   "I don't think I ever would have dreamed that Alabama would be in the position that we are in now," admitted Alabama Secretary of Commerce Greg Canfield. "Our economy is much more diverse, the automotive sector is substantial in our state and we really have become in Alabama, an international juggernaut as it relates to exports."