Not had much chance to play but not managed to find the full Nurburgring? Anyone?
If you go to special events and choose the amg challenge then you will do the ring in four stages in merc 300sl coupe with a chance to win medals and good cash, if you complete the four stages then you get to race the whole track against other mercedes cars from an aclass to black amg
Funnily enough completing that challenge does not un-lock the ring, i believe you have to reach level 13, will let you know shortly as i am at level12 at the mo