Be more worried about card with devices such as OnStar/UConnect/Sync/Entune, where the remote access system is tied to the CAN bus.

If you don't have a system such as OnStar, a hacker would have to physically access the vehicle to hack its systems. Note that ANY vehicle since 1996 can be hacked to at least stop running. The OBD2 and CAN standards ensure this by standardizing the way sensors and computers in the car talk to each other. Without a wireless network, a hacker would have to access the vehicle, tap into the network, and install hardware to allow him to perform the exploits either locally or remotely. In either case, it would be significantly more challenging and just as obvious as say cutting the left front brake line or cutting the wiring to the fuel pump.

The hacks these guys showed off on the Prius a couple years ago were really irrelevant until automakers started making remote access a common feature (and apparently not very securely). Welcome to the information age!