I'm trying to unlock car mod parts by winning in races and tried to use different cars but I get beaten no matter if I set custom cars off. What car is best? What am I missing?

You mean cheat? Try Google... you will get various methods of cheating on the came.

You don't need to cheat just get a friend and do the airport track sorry can't remember the name its an 8 shaped track it takes 30 seconds 1 lap easy win alternate and repeat you also get small amount of money and xp