I never started 8th in career, so to get this I went into in an Arcade room on Xbox Live. I created a race using 7 AI opponents, put it on one lap, and for the starting grid order I set it to Lobby points so that the higher you finish in the race, the closer to the front you'll start. When you start the race, lose on purpose and come in 8th. In the next race, you'll start in the back. Since you have the lowest Lobby points, you'll start in 8th regardless of what kind of car you pick.