You actually win the car by winning 5 games in a row. I scored over 320 multiple times in Fast Money and never won the car. I finally won the car after winning the $20,000 prize 5 times in a row in a classic game. I played solo. I'm not sure that you have to win the $20,000 prize 5 times in a row or not or just win the 5 games. If someone can confirm this I will update the solution. When I won the car I actually did pretty bad on the Fast money round and only scored 232. It was however my 5th consecutive win. You can play on Easy. Or mix easy with different difficulties. I had played other difficulties to finish those achievements as well.
Hope this helps.