There is really no right or wrong place to install tweeters in a vehicle. Most of it comes down to what sounds best to you. Experiment with different positions before you decide on the installation. With the tweeters wired, use masking tape to position them in different areas and listen to them while sitting in the driver's seat. Most tweeters can be angled once they're installed. This provides options for where to put them. For instance, you can install the tweeters in the door panel down low and then slightly angle the tweeter up toward your ears. In this way, you'll get the full directional sound but can still install them out of the way and out of sight. However, some cars have obstructions or different designs to common areas, such as door panels or kick panels, and you may find you can't hear the tweeter as well if you install them in certain places. Temporarily taping them into place lets you get an idea of what areas will and won't work for ideal listening.
Traditionally Best Spots
There are a few areas of the vehicle that usually are ideal for installing tweeters. You should still experiment with these areas to find what works best for your particular vehicle, but if you're at a loss for other installation options, one of the more popular spots will work just fine. The most often used area is on the front A-pillars on each side of the windshield. There are no obstructions across the front of the windshield, and most pillars are the ideal width and size to mount a tweeter. Also, because the pillars extend up and down the windshield, you'll be able to mount the tweeter at about ear level and angle them toward you from each side. This will give you clear, unobstructed sound from the tweeter. If you'd rather keep the tweeters out of sight, consider mounting them in the door panel next to the door speakers that most vehicles feature. If the door already has speakers installed in it, there will be enough room between the interior panel and the door body panel for wiring and the tweeter component. While you won't get the direct line of sound you would from mounting in the pillars, you can still angle the tweeter up and having it out of sight in the door panel will give a clean look.
Tweeters are the component of a speaker system that broadcasts high-range sounds. Whereas the subwoofer is for low-frequency bass, the tweeter is for the highest frequencies, such as some vocals or higher-pitched instruments. If you're installing a car audio system that has tweeters not built into the rest of the speaker, it's important to decide on the best place to install them to get the best sound.