General Motors' Cadillac Motor Division launched a direct mail promotion on Tuesday. Some 500,000 potential Catera customers will get postcards from a fantasy road trip with four celebrities. On the road for Catera: comedian Dennis Miller, actress Annie Potts, sportscaster Al Michaels and ex-Olympiad Florence Griffith Joyner. Participants can win a variety of prizes, including a new Catera, which goes on sale this fall. D'Arcy Masius Benton & Bowles, Bloomfield Hills, Mich., along with DMB&B's Clarion Marketing & Communications, Greenwich, Conn., handles.