They are not that separated. Some modern cars have the entertainment system take note of the speed of the engine and adjust the volume accordingly. Some flip the infotainment display to a backup camera when the transmission is indicated to be in reverse.

This is in addition to some cars adjusting the side mirrors when in reverse. I remember reading of an exploit that took advantage of the fact that the security system was on the same network in a particular car and thieves were able to crack off a side mirror and inject an "unlock" command. Does anybody have a reference for that?

They are typically connected, but (kind of) sandboxed. A small ECU acts as a bridge between the infotainment system and the CAN bus.

These things not always work very reliably. A guy from a supplier once held a security talk at our university. They basically filter out unwanted messages, but the manufacturer often don't configure them very restrictive, but use black list approaches.

They probably are completely separate, but the use of wireless protocols is not necessarily confined to the entertainment system. For example, wireless tyre pressure/temperature sensors are installed in many cars and can be read with SDR hardware[1]. It's not too much of a leap to assume that false readings could be injected.

[1] http://sandiego.toorcon.net/2013/10/07/dude-wheres-my-car-re...