When a story about this was up earlier there was a link in a comment to a site with an interview with a couple of engineers who had developed an app for a laptop that could be connected to the tech readout plug in a modern car and deliver a packet to the system that would made the electronic functions accessible remotely. In ssome of these cares nothing is directly connected, no hydraulic brakes or steering, all electronic. The brake pedal and the steering wheel are only switches that input to the computer on board.
I think Nissan, with their Infinity seris, is the only company that has steer-by-wire with no mechanical link between the steering wheel and the turning wheels. I don't think they're in production yet. So, right now, I don't think there is a single production automobile out there that could be steered by remote by just plugging into the vehicle's computer. Maybe somehow, they could do it with the ABS/stability/traction control systems, but I doubt it. They'd have to install some type of servo to override the mechanical link from steering wheel to wheels.