Need for Speed: Carbon was a game that was originally released in 2006. A follow-up of the already popular Need for Speed franchise, Carbon added many features, including wingmen and custom body parts. The BMW MW GTR was the featured car on previous versions of Need for Speed, but it is still possible to unlock and use in Carbon.
Select "Reward Cards" from the main menu of Need For Speed: Carbon.
Select and complete the reward cards in order. Each reward card has a set a tasks to complete before the reward on the card is given to you.
Complete the final reward card to unlock the BMW M3 GTR. You can now use it any game mode, including "Career."