Summarize:

Be sure to do this to avoid short-circuiting the electrical system during the installation, which could lead to fire or physical harm for you. For instructions on disconnecting the battery, see How to Disconnect a Car Battery. Be careful to remove all screws before trying to pry off the trim or you may break it. For some cars, you may need to remove several pieces of plastic trim, usually working from the bottom up. If you need to remove trim that includes any knobs or drawers, remove them before trying to pry off the trim. Use your hands or a pry tool to pry off each piece of trim. Pry tools are specifically for this purpose and will not damage the trim pieces. If you need to remove any components before being able to access the stereo, do so. Disconnect components that are wired to the car. Take a picture of how each is wired for later reference. Different cars may have varying elements securing the stereo in place. If the stereo is held in place by screws or nuts, loosen them with the appropriate tool (screwdriver or nutdriver, respectively). If the stereo isn't held in place by screws or nuts, you'll need to use a radio-removal key. This tool is commonly necessary in Ford vehicles. Radio-removal keys (sometimes also referred to as radio-removal tools) will typically either be in an elongated horseshoe shape or will have a circular shape at one end and a notched shaft at the other. They're available at most auto-parts stores. Insert the keys into the two small slots in the face of the stereo. You will release a mechanism holding the stereo in place. Slide the radio-removal keys into each slot again until you feel the stereo loosen from in its housing. You should then be able to pull out the stereo relatively easily. You may want to use needle-nose pliers to grip the edge of the stereo and help you pull it out. Pull it gently, and if the stereo doesn't come out easily, double check that you haven't missed any components that may be holding it in place. This is an important step because the photo will serve as a reference later when you're wiring in the new stereo. You'll see a series of wires connected to the back of the stereo, and you'll need to disconnect each of them. First unplug the antenna wire, which will typically be a thicker wire plugged in separately from the rest. Once it's unplugged, you should be able to move the stereo around more freely. Next unplug the each of the wire harness connectors. There will typically be several of these and you can recognize them because a series of wires will feed into each one. The plastic piece into which the wires are fed should have either a tab or a button you can push, which will release the harness.
Set the parking brake and disconnect the negative cable from your car battery. Unscrew any screws that are securing the trim in place. Remove the trim. Pull out any necessary components. Loosen the stereo. Pull the stereo out of the panel. Take a picture of how the stereo is wired. Unplug the stereo connections.