The first step in installing a car subwoofer is carefully disconnecting the car battery, starting with the negative terminal. Afterwards, run the wiring from the amplifier to the head unit of the stereo underneath the floor trimmings. If the subwoofer is in the car trunk, drill a hole in the wall leading to the trunk, and run the cables under the seats, securing them with clips or ties.
Next, run the power cable from the amplifier to the battery using the most direct path available. If a rubber gasket leads from the engine area into the dashboard, use this space to run the cable. If there are no available holes to run the power cable, drill a hole through the firewall, and fit a rubber grommet to avoid damaging the cable with the sharp edges of the hole. As you run the power cable through the hole, keep it away from important devices such as pedals and car controls.
Attach a ground wire to bare metal on the car's chassis, and screw in a seat bolt to hold it in place. Reconnect the car battery, beginning with the positive terminal. Be careful not to over-tighten the connections. Start the car, and play the subwoofer at a low volume for several hours to improve the subwoofer's quality.