I get this "Special Car" after some cheats, look that;
1 - Add the BMW M3 GTR in your garage (with a tool, before finishing the game)
2 - Try to add some custom piece on the car (body, skirt, wheel,...)
3 - Look in your garage, you get another BMW M3 GTR without body, only Wheel!
Tips: for helping you to drive this car, add a Spoiler (see video) (less info)