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Nearly all speakers in the interior of a car will be covered with some sort of protective paneling or grill. Before the speaker can be modified or replaced, this barrier must be removed. Pry the grill off with a suitable tool, like a flat head screwdriver, removing any bolts or screws that are holding it in place if necessary. The work you'll have to do to access your car's factory speakers will vary from car to car. In worst case scenarios, for example, you may need to remove seats, crawl into the trunk to access important bolts or wires, or even remove entire door panels to gain access to the speakers. Note that the speaker is usually, but not always, attached to a wiring harness, so take care not to rip it out when removing it. You may also find that you need to unscrew one or more small bolts and/or chip at any adhesive foam or glue that's holding the speaker in place. If you think you'll need to re-install the factory speakers in the future (for instance, if you sell the car), don't forget to save any screws that you remove! Usually, connecting your new speaker is a fairly simple matter of plugging your speaker's wiring harness into the car's wiring harness. However, if your car doesn't have this simple type of connection, you may need to connect your speaker with a soldered or crimped connection.  Make sure you match the polarity of the car and speaker's connections. Usually, the speaker's positive terminal is the larger of the two and is marked with a "+" or a small dot.  Electrical tape can be a risky choice for wire connections, especially in the dashboard, as changes in temperature can weaken the tape and lead to problems down the road. Now that you've connected your speaker, it's important to test the connection so that  you don't have to waste time later to fix a problem. Re-connect the battery's negative terminal and turn on the car's radio or stereo. Listen for sound coming out of your new speaker or look for visible vibrations at high volumes. If your speaker won't work, this means that there is a problem with its electrical connection. Once you're confident that your speaker works properly, secure it in its seat in the door or dash. If you're lucky, your new speaker will fit in the factory speaker's housing. However, your speaker may require the installation of a specially mounting bracket (usually included with the speaker itself), drilling new screw holes, and/or using adhesives to hold the speaker in place. Refer to the instructions included with your speaker. . Subwoofers are responsible for the ultra-low, "booming" bass sound that some car owners idolize. If your car came with factory subwoofers, installing new woofers can be as easy as seating them in the existing housing and connecting them to the car's wiring harness. If your car didn't come with factory subwoofers, however, or you'd like to install additional ones, your task may be much harder. You may need expand your stock woofer's existing  mounting holes or make significant modifications to the car to house large woofers. For instance, many people who want to add multiple woofers to their car custom-install a panel in the trunk to house the woofers.  Subwoofers often have fairly large power demands and complicated wiring schemes. You may want to buy and install a separate amplifier wiring kit to simplify the process of wiring your subwoofers. If not, you may need to connect the woofer directly to the battery and the car's stereo and ground the woofer manually. As with woofers, tweeters, which produce high-pitch frequencies, can be easy or difficult to install based on the factory components of your car. If your car came with tweeters, you may only need to install the new ones in the existing housing and connect them to the existing wiring harness. If, however, there are no spaces to install the tweeters, you may need to make your own (or expand existing ones, use a mounting bracket, etc. if existing housing is insufficient). Luckily, tweeters are much smaller than woofers, so the adjustments you'll need to make will be minor in comparison.  As with woofers, if your car didn't already have any tweeters, you may need connect the tweeter directly to the battery and stereo and ground the tweeter to the car's body. When all the components of your new speaker system have been installed, tested, and securely mounted in the car, you may replace any speaker grills or panels that you had to remove to install the speakers. Make sure you've kept any screws that you had to remove to take the grill or panel off so that you're able to re-secure them properly. Congratulations - your new speaker system is ready to use!

Summary:
Take any panels or speaker grilles off. Remove the factory speaker. Connect the new speaker to the car's electrical system. Test the speaker. Secure the new speaker. Install and test any subwoofers Install and test any tweeters. Replace all panels and speaker grilles.