Actually in the game somewhere (I thing it is "Goodies"), there are hints how to unlock cars/vehicles. The usual thing is like : 1. Do a mission without causing property damage. 2. Do a mission within a certain time, like beat a mission within 180 seconds or something. 3. Do a certain amount of property damage, but still beat/finish/win the mission. Those are the usual ones. I still remember that the last mission #13 in the building using a bike is the hardest to beat within a certain time. But it unlocks the best car WRC Rally with Manual Transmission, if I am not wrong.