Every so often while out exploring the world of Hell, you will encounter an APC driving around, which is shown on your mini-map as the Wages symbol used in the game. Whenever you start shooting it, its health will appear above it and the vehicle will likely take off. Deplete enough of the APC’s health and it will stop, with all of its occupants exiting.
You do not need to kill all of the demons that exit the APC, because all you need to do get in and then drive it to its destination. The destination differs depending on where you are located on the map when you steal the APC, but know that you will get a full three bars of Notoriety whenever you enter the vehicle.