Even cars with fancy-looking integrated fascias often have DIN or double DIN bracket behind which actually holds the audio unit. In those cases it's a matter of getting or having made, a custom fascia panel. Depending on the car it might be something small, or something as big as the entire center console.

The physical fitment, in skilled hands, isn't something terribly difficult to overcome. It's when the audio unit is integrated into the car's computer system that you might be looking at a much more involved project.