Adding a subwoofer is one of the best ways to improve the sound and presence of a car sound system. Contrary to popular belief, it is not necessary to have a huge speaker box and two or more subwoofers to upgrade your system. Because low frequencies are non-directional, using a single subwoofer will not impact the stereo separation, and you can place it under the driver's seat or in some other unobtrusive location. Active or self-powered subwoofers are available as an add-on component to an existing car audio system, but these tend to be underpowered. Using an unpowered or passive subwoofer in conjunction with a separate amplifier to power it is a big step up, while remaining a reasonably simple upgrade.
Find the line-level subwoofer output on your head unit or crossover. If there is only one connection, it is a monaural output; if two, it is stereo. Connect an RCA cable to the output. If your sub output is stereo, connect two RCA cables; one to each output.
Choose the amplifier you want to use. If you have stereo subwoofer outputs, you may easily use either a bridged or stereo-only amplifier. If there is only a monaural sub connection, you have no choice but to use a bridgeable amplifier.
Connect the free end of your RCA cable or cables to the inputs of the amplifier. A bridgeable amp will have a switch to place it in the mono mode. If you will be using it as a mono amp, consult the owner's manual supplied with the amp for directions to properly place it in mono mode.
Choose your woofer. If you will be using your amplifier in stereo mode, you will need a woofer with dual voice coils. This provides two sets of connections in one speaker, so that it is electrically two speakers. If your amp is bridged, a standard woofer with one voice coil will suffice.
Connect the speaker. Make sure of the polarity; the red or + terminal on the amp should be wired to the red or + terminal on the speaker. Likewise, connect the black or - terminal on the amp to the corresponding black or - terminal on the speaker.
Connect the power and relay circuits as indicated in the instructions that came with the amp. Check all your wiring, install the fuse in the power supply line and enjoy a whole new dimension in your music.
Always disconnect the negative battery cable before working on the power supply wiring to your amplifier.