A 33-year-old man riding in a Mercedes-Benz was killed when the car slammed into an MTA bus in Brooklyn, cops said Tuesday. Alex Caba-Guitierrez was ejected from the speeding red Mercedes after its driver blew a red light and plowed into the B48 bus at the corner of Harrison Ave. and Lorimer St. in Williamsburg about 9:45 p.m. Monday, cops said. Caba-Guitierrez was thrown from a backseat .