BMW owners who want to create a more complete sounding car-audio system can add aftermarket speakers, including subwoofers. The subs provide the best option for adding bass and lower tones of music to any car-audio system. The subwoofers require a stronger signal from the head unit than the other deck and door speakers. Add an amplifier to boost the signal from the BMW's stereo deck or choose powered subwoofers, which are equipped with built-in amps.
Lift the BMW's bonnet and loosen the bolts that secure each of the battery cables to their respective battery terminals with a 10mm wrench. Lift the negative battery cable from the battery and set it aside to prevent shorting the system during the installation. Lift the positive battery cable from the positive battery terminal. Slide the ring terminal from the amplifier wiring kit onto the positive battery lead. Place the positive battery cable clamp on top of the ring terminal and tighten the bolt on the clamp with the wrench.
Remove the fuse from the fuse holder on the ring terminal. You will need to insert the fuse as a final step so keep it handy. Insert the power supply cable (included in the amp wiring kit) into the end of the ring terminal. Route the cable through the BMW's firewall (separating the engine compartment from the passenger cabin). Move to the inside of the vehicle.
Pry the trim panel that spans the length of the BMW's dashboard away from the dash with a trim panel removal tool. Remove the panel and set it aside. Remove the screws beside the stereo assembly with the Phillips-head screwdriver. Remove the panel that surrounds the stereo and climate controls. Remove the two screws on either side of the stereo bracket with the screwdriver.
Slide the stereo from the BMW's dashboard stereo dock and disconnect the stereo wiring connectors from the back of the unit. Press the release mechanism on each stereo wiring connector before pulling them from the outlets on the rear panel of the stock stereo. Connect the stereo wiring connectors to the wiring harness adaptor leads. Plug the adaptor's leads into the appropriate outlets on the back of the head unit.
Plug the RCA adaptor into the amplifier leads of the wiring harness adaptor. See the wiring harness adaptor's User's Manual for proper connection locations or follow the colour-coding or other indicators on the leads themselves. Plug the RCA patch cables into the RCA adaptor. Feed the cables through the back of the BMW's stereo mounting dock and down to the floor of the vehicle on the passenger's side.
Slide the stereo back into the stereo dock. Replace the screws and trim panels onto the dashboard. Route the RCA patch cables along the passenger's side of the vehicle to the rear cargo area (the boot). Lay the power supply cable from the driver's side of the BMW to the boot.
Place the powered subwoofer in the boot. Connect the grounding wire from the grounding point on the sub to a bolt or screw in the boot of the vehicle. Any bolt or screw less than 18-inches from the subwoofer and directly connected to the frame of the BMW will suffice. Plug the signal and turn-on lead cables into the corresponding outlets on the powered sub. Insert the power supply lead into the power input on the subwoofer. Return to the battery.
Place the negative battery cable clamp around the negative battery terminal. Tighten the cable's bolt with the wrench. Slide the fuse into the fuse holder on the ring terminal and close the BMW's bonnet.