For my car, I have a reverse camera / display which also links through the GPS / headunit. So in my case, removal of the head unit would have disabled the reverse camera, blanked out the screen for tall the environmental info / trip computer and rendered most of the heater controls inoperable. So it really depends on how integrated everything is, I've ripped out Jap units before nad easily replaced them with Euro ones but for some cars it's almost impossible unless you want to loose a lot of functionality....