Although installing a 4-channel amplifier may seem like a daunting task at first, it's actually a straightforward process as long as you plan out the installation. The high volume of wires and interior work required for a clean installation can create a headache if not handled properly, so it's important to have a wiring diagram handy and to separate tools, nuts and interior panels.
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Ground the Amplifier
Disconnect the negative terminal on the car battery for safety purposes. Most car battery terminals require a monkey wrench to unscrew the terminals.
Place the 4-channel amplifier in a dry, well-ventilated area such as the trunk.
Connect the ground wire to the amplifier. Amplifiers differ in the way you connect certain wires, so refer to your amplifier's wiring guide for the proper procedure. Most likely it will only require some type of screwdriver.
Locate a stable, metal fixture in your car to ground the amplifier. Often the best place to ground an amplifier is on a screw or nut in the interior. Unscrew a nut and place the ground output at the base of the nut and screw the nut back on over the ground output.
Install the Power Wire
Connect the power wire plug-in to the amplifier. The amplifier's power input should be clearly labeled "Power" or something similar.
Search underneath the dashboard for a hole to run the power wire through to the car battery. You will likely have to peel back carpeting and potentially cut a hole in the firewall using an X-acto blade.
Run the power wire to the front of your car and slide it through the hole to the engine bay. Attach the wire's output lead to the car's positive battery terminal. Zip tie the wire to solid metal objects (no wires) inside the engine bay if necessary to keep the wire taught.
Attach a fuse to the power wire. Be sure to attach it within 18 inches of the car battery. Refer to the instruction manual that came with your power wire or fuse. Some fuses clip onto the wire, some are already installed on the wire and some must be assembled.
Seal the hole to the engine so water doesn't drip down the power wire and inside the car. Use caulk or a rubber grommet.
Connect the RCA Cable and Speaker Wire
Connect the RCA cable to the amplifier's input by plugging it in. It should be clearly labeled "RCA."
Remove the head unit using the head unit removal tool that came with your deck.
Plug in the other end of the RCA cable to the RCA output on the head unit. Be sure to run the RCA cable along the opposite side you ran the power wire on to avoid static.
Attach speaker wire to the terminals in the back of the speakers that you wish to power with your amplifier. Strip approximately 3/8 of an inch off the speaker wire using wire cutters to reveal the copper wire underneath the wire cover.
Connect the opposite end of the speaker wire leads to the amplifier, stripping them the same way you did in step three. Be sure you wire the correct speakers to the proper channel. For instance a speaker sitting on the driver's door will want to be plugged into the left channel of the amplifier.