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Slide the negative terminal back onto the port on the battery and push it down as far as you can. Tighten the nut with a socket wrench to secure the terminal back to the battery so you're able to use your vehicle again. Turn the key in the ignition to start your vehicle and check if the power light turns on for the amp. If you don't see the power light turn on, shut off your vehicle and disconnect the negative terminal before checking all of the power connections. Look for a knob or screw that's labeled "Gain" or "Amp Sensitivity" on the front of your amp. If there's a knob, turn it counterclockwise by hand to lower the gain. If there's a screw, turn it counterclockwise with a screwdriver until it doesn't go any further. The gain controls how loud the amp will output the audio from your stereo. Put a CD in the stereo or connect an MP3 device so you can play music. Choose a song that you know well and has crisp, clear audio so you can hear distortion easily. Keep the song on repeat so you can continue listening for audio interference. You can also use the radio, but make sure the station comes in without any static, or else it can be hard to tell if it's caused by the amp. Turn the volume knob slowly on the stereo to increase the volume. Keep turning the dial until you hear static or interference coming through your speakers. Find the volume level right below where the distortion starts so you can adjust your vehicle's audio accordingly. You won't hear the music start playing yet since the speakers run through the amp and the gain is turned down. Rotate the gain control clockwise by hand or with a screwdriver so you can hear the audio through your speakers. Continue raising the gain setting until you have the audio as loud as you think you'll play it in your vehicle. If you hear any distortion or interference, turn the gain down slightly until you don't hear it anymore. Once you're happy with the gain levels, turn the volume down at your stereo. Whenever you want to make volume adjustments after setting the gain, you can use the knob on the stereo head. Push the wires for the stereo head back into the hole and position it so the mounting holes line up with the dashboard. Screw the stereo into the dashboard so it's secure. Line up the trim panels around the stereo and on the sides of your vehicle and push them in until they snap into place. Keep attaching the rest of the trim pieces to finish the installation.
Reconnect the negative battery terminal to test the amplifier. Turn the gain on the amp down as far as it can go. Play clean audio that you're familiar with through your stereo. Raise the volume on the stereo head until you hear distortion. Set the gain controls until the music is as loud as you plan on listening to it. Reattach the trim pieces and stereo head when you're finished.