A car could be self-driven while not connecting to any external networks. It still can get fresh traffic or weather data, but with those, you can't take over a car to make it a weapon.
Think Reflected XSS: weaponized string hidden in map data (road name even) in advance, is unwittingly integrated into offline map, triggers when a self-driving car tries to parse the data for that road segment, using a vulnerable library. Antyhing that uses network-contributed data is network-connected, even if the link is currently inactive (Case in point: Stuxnet).