The Special Vehicle Activity is a side mission activity in the Karthstan project.
To find special vehicles, you must find them. They're usually escorted by a convoy and attempting to steal it will result in a Heat Level 4, Rico must lose the heat with the vehicle without leaving or destroying it, they can be easily found with a symbol of a steering wheel.
These special vehicles are usually easter eggs to other games or pop culture. Here's a list of vehicles. The Reward for stealing all special vehicles is a ultimate special vehicle.
MORE VEHICLES COMING SOON™