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 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...