Subwoofers are speakers best used to provide balance to your vehicle's audio system, specifically the lower tones of music and bass. Adding the subs to your the audio system requires a boost in signal strength to power the larger speakers. You can use an aftermarket amplifier to boost the signal from the vehicle's head unit or install powered subwoofers that have built-in amplifiers. If you choose to not enlist the services of a car stereo installation professional, purchase the necessary adaptors and wiring and do the job yourself.
Raise the vehicle's bonnet to access the battery. The powered subwoofer requires an independent power connection to the car's battery.
Loosen the bolts holding each of the battery cables to their respective terminals with an adjustable socket wrench. Remove the negative battery cable clamp from the negative terminal. The negative cable is commonly black and connects the battery to the frame of the vehicle for grounding. Place the cable away from the battery terminals.
Remove the positive cable clamp from the terminal post. Place the ring terminal included in the amplifier wiring kit onto the positive battery terminal. Place the positive battery cable clamp back onto the terminal post, over the ring terminal, and tighten the bolt on the clamp with the wrench.
Remove the fuse from the slot on the ring terminal and keep it readily available for reinsertion once the subwoofers have been installed. Insert the power supply cable (included in the amp wiring kit) into the input on the side of the ring terminal. Feed the power supply cable into the vehicle's cabin through an already established opening in the firewall separating the engine compartment from the passenger cabin. If no path exists or is not available, move to Step 5; otherwise proceed to Step 6.
Drill an opening in the firewall with a drill and drill bit at least one size higher than the size of the power supply cable you are using. Make the new opening on the driver's side of the vehicle to make routing the cable to the subwoofer in following steps easier. Place a rubber grommet into the new opening to protect the power cable from the sharp edges of the new hole. Feed the power supply cable through the new opening and into the inside of the vehicle.
Move to the passenger's cabin of the vehicle. Remove the factory stereo from the dashboard. Follow the instructions for head unit removal found in the vehicle repair manual. The necessary tools and procedures will vary based on your vehicle make and model. Common steps include removal of the dashboard or dashboard panels and extracting the stereo unit from the vehicle's stereo mounting dock.
Disconnect the stereo wiring connectors from the outlets on the rear panel of the head unit. Plug the vehicle's stereo wiring connectors into the wiring harness adaptor leads. Follow the colour-code or abbreviations on the wiring harness for guidance. Plug the wiring harness adaptor's lead connectors into the corresponding outlets on the back of the vehicle's stereo.
Connect the patch cables to the wiring harness adaptor leads labelled for amplifier input. Consult the wiring harness adaptor's manual if the proper connection points are not clearly identified on the leads themselves. Feed the patch cables through the back of the vehicle's stereo mounting dock and down to the floor of the vehicle, ideally on the passenger's side.
Replace the stereo into the dashboard. Follow the guidelines in the repair manual for stereo installation. Replace any of the dashboard panels that were removed to remove the stereo in Step 6.
Route the power supply cable and the patch cables along opposite sides of the vehicle to the rear cargo area or boot. Keep the patch cables and power supply cable separate to prevent signal interference. Tuck the cables into the panels along the sides of the vehicle with a trim panel removal tool. See the repair manual for locations of panels that will make for ideal places to conceal the wiring.
Place the powered subwoofer into the boot or rear cargo area of the vehicle. Connect the subwoofer's grounding port to a screw or bolt near the sub with the grounding wire from the amplifier wiring kit. Any screw or bolt less than 20 inches from the subwoofer and directly connected to the vehicle's frame will serve as an adequate grounding source.
Insert the power supply cable into the power input on the subwoofer. Connect the patch cables to the signal inputs on the sub, normally on the same panel as the power input. Return to the engine compartment.
Replace the negative battery cable clamp over the negative battery terminal. Tighten the bolt on the cable's clamp with the wrench. Place the fuse back into the slot on the ring terminal and close the vehicle's bonnet.