Could some one please tell me how to do take down a special car in whatever track on the challenge sheets

It's the car behind you once you start a race. Do it on the track the challenge sheet is on.

I.E. Challenge sheet for White Mountain.
Challenge - Take down a special car.
Track - Any track of white mountain that is a race.
Rank challenge can be beaten on - any rank as long as it's in white mountain.

Usually it's a fancy car like the F1 style racer of the "street rod" antique-looking car.