You will most likely get this if you play the campaign normally, but if you are bad like me, here is an easy way to grind them out.
* Go to Local Multiplayer (no need for 2nd controller).
* Press "Add Player" (you will need to add a player with the +new option first time) and press
* Press Y to toggle AI players off (you can see if it's on or off at the bottom centre of the screen).
to add a track. I would suggest Ovals -> Epic Oval. It is easy and takes 5-6 secs.
* When added, press
to start the game (you can see the X prompt on bottom of screen)
* You will need to select the type of car to use (I suggest Rally Car) and press
The race will start and you will have no opponents. Complete 5 easy laps at your own leisure and it will count as a win.
At the end, just restart the race until you have 100 done.