Partly like what @Mara says, but more important; in the newest vehicles there's a permanent 3G connection and GPS tracking, which sends a distress signal coupled with location when an accident happens for example. The same thing works in the opposite way of course, the car manufacturer and their licensed reps can remotely diagnose the car's systems and send software updates to the ECU.

What is more alarming IMHO is that such features will be made mandatory in the future, so that all new vehicles will be equipped with them. The intention is good of course but it will indeed plant a new attack vector to the vehicles.