Summarize the following:
Turn off the car's ignition before opening the hood and interacting with the battery. Use a wrench to loosen the nut on the negative end of the battery (marked by a “-”), then detach the black negative cable from the battery. Doing this will help prevent a possible short circuit during the tweeter installation process, which could damage your car's other electronic components. For safety, wear insulated gloves that will protect you from any charge the battery might give off. You should also wear safety goggles when working with a car battery, in case any of the corrosive materials inside the battery begin to leak. If you plan to mount your speakers under your dashboard or inside a car door, you'll need to remove the dashboard panel or door panel before you can proceed. Use a screwdriver to remove any screws that are keeping the panel attached to the body of the car. Then, use a wide, flat putty knife to pry the panel away.  Note that this step isn't necessary if you plan to surface mount your tweeters. The screws on a door panel are most likely located underneath the door lever and underneath the arm rest. The locations of the screws on your dashboard vary based on the make and model of your car. Check your user's manual for specific instructions on how to locate these screws and remove your dashboard. In a bottom mount installation, the tweeter is placed underneath an existing speaker grille, meaning you don't need to drill any new holes to install it. Simply use a wrench to screw the tweeter into the factory-installed holes underneath the grille. Note that if your car doesn't have holes underneath the speaker grille that you can screw your tweeter into, you'll need to drill those holes yourself. These will only need to be about 0.25 inches (0.64 cm) deep or so, in order to give the screws enough space to be securely attached. Flush mounting a tweeter in the door panel is the most common installation method. First, drill a hole into the door panel that is almost exactly as wide as the tweeter. Then, slide the base cup (which holds the tweeter in place) into the hole so that it is snugly secure in the panel. Finally, attach the tweeter to the base cup.  Refer to the instructions that came with your base cup to see how precisely to attach the tweeter to it. Most cups simply require you to pop the tweeter into it, while others may have you use a screw to attach it. Ideally, you should drill into the door panel after it has been removed from the door itself to avoid inadvertently damaging anything behind the panel. If your door panel has an existing speaker grille in place, this is the best location for your tweeter. However, if there's no grille on your car door, then feel free to install your tweeter wherever you want high-frequency sound to come from! This method of installation is called “flush mounting” because the top of the tweeter sits flush with the surface that it's mounted on. Use a power drill to drill a small hole in your car's speaker grille; this will be the hole that you'll stick the speaker wires through to wire your tweeter to the crossover. Then, use a screwdriver to screw the base cup into the grille with a pair of screws. Then, attach the tweeter to the base cup by sliding it in so that it's snugly secure inside the cup.  Avoid drilling the hole in the grille to deep, as you may cause unwanted damage to the speaker system. You only need to create an opening in the grille just big enough for speaker wires to pass through. Surface mounting is a useful installation method for when there aren't a lot of deep places in your car where you can drill new holes for installing tweeters. The most common location for surface mounted tweeters is on the “A” pillar, which stands between your windshield and your front door window.

Summary:
Disconnect the negative terminal from the car battery to cut the power. Remove the necessary panels to gain access to your car's speakers. Install the tweeter in the dash for an easy bottom mount installation. Mount the tweeter in the door panel to put the sounds closer to you. Opt for a surface mount if you can't put the tweeter anywhere else.