You have to get all the vehicles in the base game. Vehicles are unlocked by leveling and completing special events. The respective condition you can see in the vehicle list, if you select the respective not yet available. Ultimately, unlocking all vehicles is only a matter of time. If you complete the entire "Off-Road Masters" career and pay attention there to the victories, you will most likely unlock the last vehicle with the last event "Final League", because by then the driver level will be sufficiently high.