As implied in Cory Doctorow's "The coming war on general-purpose computation," modern cars are really computers with an engine and wheels. This event has proven that computer security breaches in cars can have real-world legal consequences for citizens. As a result, there may be a market in a hardening/privacy guide for new cars, similar to the kind sysadmins use to harden Internet-facing servers. Alongside your standard hacker-types, a guide like this probably has a market in survivalist/conspiracy circles.
The guide could explain how to change the code in the car's alarm transmitter as well as how to remove devices with privacy implications like OnStar.
Modern cars are really a LAN with an engine and wheels. You need to decide how secure each computer on the network needs to be or whether it is enough that you need be able to get into the car to get at the diagnostic connector. There was a case a while back of one model of car being easily stolen because the cables to the ABS system accessable from the wheel arches.
There has also been talk of allowing wireless access to the in-car network in order to allow cars to be driven much closer together in high-occupancy lanes.
The guide could explain how to change the code in the car's alarm transmitter as well as how to remove devices with privacy implications like OnStar.