“Want to take a mind excursion? How ‘bout a little diversion? The hippest of all trips. The return to reality. Well, join me.” With that introduction, William Mercer, or Rosko to free-form radio fans, would open his shows that showcased a broad range of music: rock, soul, folk, and jazz. Mercer, who grew up on New York’s 114th Street and Manhattan Avenue, read poetry by Kahlil Gibran and delivered impassioned monologues against the My Lai massacre and the Kent State shootings