Determine what side of the vehicle the battery is on. Disconnect the negative battery terminal. If you have a radio with a security feature (such as a Delco-Loc, Eclipse ESN, etc.) make sure you know the reactivation or security code before disconnecting the negative battery terminal.

Find a spot to run the power wire through the firewall. Try to find an empty rubber grommet that hides an unused factory hole. You can just put a hole in this grommet and run your wire through.

If there is no factory hole you will need to make one. Get out the power drill and steel drill bits. Before drilling make sure you know where the bit will end up once it's through the firewall. I usually make a small pilot hole and then increase the bit size until the hole is big enough for my wire. If you have a stepped bit (i.e. Uni-Bit) that would be great to use (see photo). I recommend them highly.

Protect the drilled hole edges from rust with paint or a similar material. Don't skip this step.

Install a rubber grommet (see photo) into the hole to protect the wire from chaffing (VERY IMPORTANT!). Caulk the gap between the inner diameter of the rubber grommet and the power wire if necessary to prevent water from running down the wire. Larger gauge wire such as 1/0 will require a specialized grommet and a much larger hole. StreetWires makes these type of two piece grommets which they call firewall bushings.

Run the wire from the interior of the vehicle, through the hole and to the battery area, being very careful to avoid pinch points and moving parts in the engine bay. It's easier to push 5-8ft of wire through the grommet than the other 15-20ft from the battery side. That's why you push the wire from the interior.

Install an in-line fuse holder (see photo) for the power wire as close as possible to the battery terminal connection (the IASCA rule is within 18" but less is better ). VERY, VERY IMPORTANT!!!!! This fuse protects your vehicle from burning up in case of the power wiring shorting out. It's not there to protect your audio equipment, it's there to protect your car. The wire between the fuse and the battery terminal will be unprotected, which is why you want this section to be as short as possible.

The fuse size depends on the current the power wire will carry. Consult the owner's manual of your amplifier(s) to find out the maximum current draw of your system. You can also contact the manufacturer and ask them for that information. For multiple amplifiers you will need to add up the maximum current draw of each amplifier. Once you know the maximum current draw you'll know what size fuse to use. For example, if your maximum current draw is 50 amps then you'll need a 50 amp fuse. Do not insert the fuse until all other connections are made! Connect the power wire to the battery using appropriate connectors for the wire and battery terminal. For more help on choosing a fuse size visit this page.

Pull back the carpet in the area you intend to run the power wire. You will need to remove the rocker panel and kick panel covers to do this.

Snake the power wire to it's final termination point (where your amp will be mounted). Be careful to avoid any pinch points such as seat tracks/door jambs, etc. Depending on your application, you can connect the power wire to the amplifier (using appropriate connectors such as ring terminals, see photo) or connect it to a power distribution block if you are using multiple amplifiers.

After you have your power wire run the way you want it, go back and use plastic wire ties to secure the wire in place. You may also want to cover the wire in the engine compartment with wire loom (prior to using wire ties). You can find this loom at auto parts stores and Parts Express.

Now run the RCA cables and remote turn-on lead (typically a blue lead but you will need to check your head unit's manual) from the back of your head unit to your amplifier's mounting location. You will need to follow a process similar to that of the power wire (steps 8 and 9). However, run your RCA cables on the opposite side of vehicle. You do not want to run power wires and signal cables on the same side of the vehicle. This could induce noise into your system. Connect the remote turn-on wire to your amplifier using an appropriate connector (such as a ring terminal). NOTE: The remote wire is what sends the signal telling your amplifier to turn on or off. It is a very important wire and your amplifier will not work without it. 

Now run your speaker wires between your amplifier and your speakers. If you need to run wires through the interior of the vehicle, make sure you run them on the same side as the RCA cables. You do not want to run power wires and speaker wires on the same side of the vehicle. This could induce noise into your system although it is less likely than picking up noise through the RCA cables.

Connect the speaker wires to your amplifier using appropriate connectors (such as ring terminals) and to your speakers using the appropriate connectors (such as female spade terminals, see photo). Most speakers have two terminals of varying sizes. The larger terminal is the positive terminal and the smaller is the negative terminal. NOTE: If your speakers were previously connected to (powered by) your head unit you will need to first disconnect the speakers from the head unit. You cannot have your head unit AND your amplifier connected to the speakers. This will cause damage to your electronics.

Now you will need to connect the negative terminal of your amplifier to the chassis ground of your vehicle. Any metal point on the vehicle should work since they all lead back to the negative terminal of the battery. This negative power wire should be short and of the same diameter as the positive power wire leading to the battery. When connecting the negative power wire to ground, you may need to scrape some paint from the grounding surface. If you have a Dremel type tool with a metal brush these work great (wear eye protection). This will ensure better contact between the negative power wire terminal and the vehicle ground. After you have the negative terminal connected to ground, cover the connection with paint, caulk or a similar protective material to prevent rust (corrosion = noise). CAUTION: If you must drill a hole in the vehicle to connect the chassis ground, make sure you know what you're drilling into. This is especially important when you are in the area of your vehicle's gas tank. A wrong move here could puncture the tank, cause a spark and set your vehicle on fire with you in it! If you are unsure then consult a professional.

After all connections are in place and all wires are secured, you may insert the main power wire fuse. If you are installing a car audio capacitor you will need to charge it first. This can also be done at the fuse holder.

Reconnect the negative battery terminal.

Congratulations! You just installed a car audio amplifier. Now you'll want to set your gains properly. See the instructions below.