Forza 4: How to unlock the Halo Warthog

In Forza Motorsport 4 there is a mode called autovista. You can look around and inside cars whilst a commentators gives you interesting facts about them.
This is where you unlock the Warthog.


First off, go to the autovista mode and there will be 20 cars that are locked and are unlocked by challenges, complete these challenges and once you have completed 19 of them then the Warthog will become accessible.