Do they really? Most recent cars I've seen have a touchscreen LCD popping up out of the dash and it all looks rather custom. Are there some kind of standards like double-DIN?
What GP complained about was that the factory-installed head units have very non-standard shapes for everything but the screen, and even then it'd be glued into some molded piece of plastic trim which has a non-regular shape in the dashboard.
You'd literally have to cut out the trim with a saw and refinish it in order to use the din/double-din standard sizes.