I know, I bought my automobile computer on Ebay for just over $50. The plug is supposed to be “universal” and works on every vehicle I own (Toyota Tacoma and Accura TL).
As far as being “impossible”, I beg to disagree. If the car is communicating to the car computer through the I2C or CAN bus. The CAN bus handles things such as timing, gas/air mixtures and when the transmission should shift. By virtue of the car computer giving this information to a central processing unit - the bus is open. There is no SECURE transaction on the CAN bus - there is no SUPERVISOR mode in a CAN bus transaction. Thus, by virtue of the CAN bus tying into the network - the network is vulnerable. One must ASSUME there is some firewall between the USER interface and the CAN bus. However, I have seen nothing in the bus protocols or interface circuits that state such a device is in existence.
At Wingcast, we did fun stuff like flashing headlights, honking horns, unlocking doors and monitoring the GPS. I could also initiate an outbound phone call. Most of that was SCP (Standard Corporate Protocol) a Ford variant of J1850. The MCP variant ran on the entertainment equipment. The safety engineer asked if the display on the radio could be used to display a scrolling message. He returned from lunch and I had "Attention K-Mart Shoppers" scrolling by. That immediately became a prohibited feature :-)
Some vehicles have remote start capability hooked into the fieldbus with remote radio access from a service center.