Below is a guide that will explain how you can install them without a crossover:
1)Get both car tweeters and get hold of two small electrical capacitors. If you don't want to buy them, you can find them in broken car amplifiers and car head units. What a capacitor will do is filter out the bass frequencies and increase the volume clarity and reduce distortion.
2)Get you capacitor and solder one leg to the positive terminal onto the tweeter and solder the other leg onto the positive end of the speaker wire, doing this for both of them.
3)Solder the negative ends of the speaker wire onto the negative terminal of both tweeters.
4)Connect the positive and negative wires onto your car speaker terminals. Repeat the same process for your other car speaker and tweeter.