This might not be what you want to here but this is what has happened to me. I have the ps3 version and I was extremely perturbed by the ghost car, however since I can't turn it off in my version( i have tried) I now don't mind it and it actually helps figure out just how late you can brake. If you don't know the track you just follow the ghost around until you are faster and then you can set faster and faster laps.
I couldn't get rid of it so I use it