I am skeptical that the car-fi aftermarket has the potential to explode the way home theater has in recent years. The same forces driving the widespread adoption of mp3 as the predominant music distribution format weigh heavily on mobile audio.
Since the enjoyment of video is an all-consuming activity, quality in home theater makes perfect sense. When we are going to watch a movie or a TV show, there is little else we can do at the same time. Because both our visual and aural senses are fully engaged, it is much easier for the average consumer to appreciate the difference in experience between a small TV with built-in speakers and a large display paired with a full surround system.
It is rare for anyone to allow themselves the same kind of engagement when listening to music. The primary decision point in music consumption is convenience. Most people listen to music while doing something else: Working, exercising, cleaning the house, and yes, driving. Often these activities are paired with a secondary activity such as holding a conversation.
In the situations where most people consume most of their music, convenience is king and quality only needs to be "good enough". Given that mp3 players and smartphones are becoming *the *entire music library for most of us (either by buying/ripping our own music or, for the next generations, through the world-wide all-you-can-eat music libraries of services like Spotify and Pandora), interfacing with these devices is quickly becoming the most important feature of car-fi.
The OEM car audio designers/manufacturers have figured this out. It's very difficult to buy a car without at least a dash-mounted aux-input jack. Most new car models have at least a USB interface, if not full-blown control systems, for iPod/iPhone. They put more capital (human and financial) into designing these convenience features than they put into quality amps and speakers. (Built-in support for streaming Internet audio is already appearing on high-end models and will likely be the next must-have car-fi feature.)
When even the Compact Disc is considered inconvenient, and its adequate measure of quality is readily sacrificed for mp3s (or worse, when considering streaming music services), I very much doubt the aftermarket car-fi world will ever be more than a niche.
All that said, the aftermarket component designers aren't doing much to convince anyone otherwise by having such user-hostile interfaces.