I am not sure if this is what you mean but....in online use when you get the cars you can use them all. In the "world tour" mode you have to unlock the car with the same body style. All the downloadable cars are basically just skins for an already unlocked/locked car.
So, go through the game and unlock the cars but completing challenge sheets. You can than use the car in world tour mode. You probably just unlocked the car that has the spike tv skin, that's my best guess.