You can make it so much better than stock with proper insulation and speaker elements. Car manufacturers often use 5€$ elements which sound like shit.
replyYou can't have a proper soundstage in a car no matter your setup, which kinda degrades a lot of music that wasn't designed for clubs. In a car it's basically background music when you're driving because you can't really give the music your full attention anyway (passive listening), where the lack of focus also masks a lot of details you'd hear on a proper stereo system (active listening).
replyEdit: to answer your question below, I like both, active and passive listening. But I wouldn't try to optimize for active criteria like details in a passive evironment like a car, so I'd spend more money on fun things like buttkickers instead of high end detail.
Obviously a car isn't a dedicated listening room. Music can be enjoyed by just listening to it and not focusing on all the nuances. Or are you one of those hardcore audiophiles who can't listen to music without a ultra high end sound system?
replyAlso soundstage can be greatly improved by placing speakers on non factory locations because manufacturers usually prioritize packaging above all else. A tweeter placed at the bottom of the A-pillar near the windshield is the worst possible location and sounds like it was put in a tin can.
Background noise in a car also has a solution, turning up the volume. Plus getting quiet tires.