If there's a quicker way to do this let me know but this is what I did:
Start career in WRC3 as a manufacturer team, set difficulty to 50, damage to none and length to short. Then you need to win the WRC3 championship, the difficulty set to 50 should make this easy. Once this is done you'll have the change to jump up to WRC2, do the same as you did previously and aim to win so you can move up to WRC, you then need to win WRC to unlock this.
Took me a couple of hours only focusing on the rallies and nothing else.
If you downvote, let me know why.
Thanks.