By Wendy Fry, The San Diego Union-Tribune (TNS) TIJUANA, Mexico — They come for the tacos. Or the music. Or the nightlife. They stay to live a less-expensive lifestyle or to retire in a casita in the sand. American immigrants living the Mexican dream can have the same hopes and goals of the Mexican immigrant in the United States: to get a little ahead or to start a new life. There may be more Americans