All new cars have diagnostic ports using open protocols. http://www.obdii.com/ has some of the details. They use the CAN bus so it would be fairly easy to attach a small microcontroller with CAN support and an 802.11 or GSM module to give you remote access.
I'm not sure how well documented the control protocols are (stopping car, turning off brakes, etc.) but it is apparent from this article that they can be discovered.
I'm not sure how well documented the control protocols are (stopping car, turning off brakes, etc.) but it is apparent from this article that they can be discovered.