Whether it's a passive crossover or an active crossover, your car audio system will never sound good without a crossover. I want to make myself very clear here, so before you get confused let's talk about what a car audio crossover is and why you need one.
A crossover is a filtering device which limits the frequencies that reach a speaker. It splits a music signal into separate frequency ranges and sends them to speakers that are designed to best reproduce each frequency range. For instance, only high frequencies would be sent to your tweeters, midrange to your mid range speakers, and lows to your subwoofer.
The passive crossover is very common. It's basically a capacitor or coil installed on the speaker leads between amplifier and speaker that stops certain frequencies from reaching a speaker. It's relatively inexpensive and easy to install.
But for your car stereo system to perform better and be efficient, you need an active crossover, also referred to as an electronic crossover. It is installed between your head unit or equalizer and your amplifiers. Your electronic crossover sends the proper frequency ranges to each amplifier. Some people run three separate amps: one for bass, one for mids and the other for highs.
Active crossovers are almost infinitely adjustable. You can vary the crossover points and adjust the level of separate speakers to achieve the best overall sound in your vehicle.
If you're serious about sound you must have a car audio crossover. Otherwise your system will sound terrible. Oh what a mess it will be when your subwoofer and mid-range speakers duplicate many of the same frequencies! How about when your mid-range speaker attempts to put out high notes that your tweeters are supposed to handle. Can you bear such grief? I can't.
And this is just the beginning of your sorrow and pain. Since low frequencies are being sent to your tweeters there will be distortion. As a result of this distortion your tweeters will be destroyed. Loud bass will eventually destroy your mid-range speakers as well. Now you've got to start from scratch and buy a new set of speakers, all because you started on the wrong footing.