DETROIT — Dejuan Ross is a veteran Toyota Motor North America executive who became head of the Lexus brand at the end of June in a management shuffle brought about by the retirement of longtime Toyota sales head Bob Carter. Ross, 50, a graduate of Howard University, has been with Toyota in a variety of sales and marketing positions since he joined the company in 1995. Before his appointment to head up Lexus, he was vice president of marketing for Toyota Financial Services.