How to Install a Car Subwoofer
To install a car subwoofer, run a power wire from the battery through the firewall to the trunk, mount your amp and connect its power, ground, and remote turn-on lead, feed it a signal from the head unit (RCA or speaker-level through a line output converter), wire the sub inside its enclosure, and set the gain and low-pass filter before you close everything up. A full install takes most of a Saturday the first time you do it. Rush the ground and the fuse near the battery and you will regret it, so I do those two things carefully every single time.
I have installed a lot of these in a lot of trunks, and the steps below are the exact order I work in. Follow it and you will not be chasing whine, blown fuses, or a sub that thumps once and quits.
What You Need
Before you start pulling panels, get everything in one place. Nothing kills an install like realizing the wiring kit is one ring terminal short with the car half apart.
- Subwoofer. An 8, 10, 12, or 15 inch driver. A 12 is the common all-rounder for a first build. If trunk space is tight, an under-seat sub is the low-hassle route.
- Amplifier. A mono (Class D) amp for a single sub. Match its RMS power to the sub’s RMS, not the peak numbers on the box.
- Enclosure. Sealed for tight, accurate bass; ported for louder, boomier output. The box has to match what the sub wants, so check the driver’s recommended volume.
- Wiring kit. Power wire, ground wire, in-line fuse holder, remote turn-on wire, RCA cable, and speaker wire. Buy the kit sized to your amp, not one gauge thinner to save a little.
- Line output converter (LOC). Only if you are keeping a factory head unit with no RCA preouts. It taps the factory speaker signal and gives the amp a clean RCA input.
- Tools. Panel removal tools or a plastic trim pry, socket set, wire strippers and crimpers, a test light or multimeter, a heat gun or lighter for heat-shrink, a drill for the firewall grommet, and a trim panel spudger so you do not gouge anything.
Planning the Install
Ten minutes of planning saves you an hour of redoing work. Sort these three things before a single wire moves.
Powered vs Passive
A passive setup is a separate sub and separate amp, which is what most people picture and what I am walking through here. A powered sub is the driver, box, and amp built into one enclosure. Powered units are simpler and great for tight spaces, but you still run power, ground, and signal to them, so the wiring part of this guide applies either way.
Matching the Amp to the Sub
This is where most first builds go sideways. Match RMS to RMS. If the sub is 400 watts RMS, you want an amp that puts out somewhere around 400 watts RMS at the sub’s impedance. Ignore the peak and max numbers printed in big font, they are marketing. An amp that is a little underpowered runs clean. An amp that is way overpowered lets you cook the voice coil if you are careless with the gain. Also check the impedance the amp is stable at (2 ohm, 1 ohm) and how your sub’s voice coils are wired, because that decides the power you actually get.
Wire Gauge
The power and ground wire have to carry the current the amp pulls. Too thin and the wire heats up, the voltage sags, and the sub sounds weak. Use the sub’s total RMS power to pick the gauge, and keep the power and ground the same size. Table is below in the tuning section area. For most single-sub builds you land on 8 gauge or 4 gauge.
Step-by-Step Installation
Work in this order. Disconnect the negative battery terminal before you touch anything electrical, and leave the fuse out of the holder at the battery until the very end.
- Disconnect the battery. Pull the negative terminal. This is not optional. It keeps you from arcing a wrench across the battery while you run power.
- Run the power wire from the battery. Crimp a ring terminal on the power wire and set it aside near the positive post, but do not connect it yet. Route the wire toward the firewall, keeping it clear of anything hot or moving.
- Get through the firewall. Find an existing rubber grommet in the firewall and push the wire through it, or drill a new hole and fit a rubber grommet so the metal edge cannot cut the insulation. Never run bare wire through bare sheet metal.
- Route power to the trunk. Run the wire down one side of the car under the door sills and kick panels, away from the factory harness and away from the signal wire, which you will run on the opposite side. Keeping power and RCA apart is how you avoid engine whine.
- Set the ground. At the amp location, find bare chassis metal. Sand the paint off down to shiny metal, drill and bolt a ring terminal to it, and keep the ground wire short, under about 18 inches. A bad ground causes more subwoofer problems than any other single mistake.
- Run the remote turn-on wire. This thin wire tells the amp when to switch on. Tap it to the head unit’s remote lead, or to a switched 12 volt source that only has power when the key is on. Run it alongside the RCA, not the power wire.
- Run the signal. If your head unit has RCA preouts, run an RCA cable from the head unit to the amp. If you have a factory stereo with no preouts, wire a line output converter into the factory speaker wires (usually the rear speakers), then run RCA from the LOC to the amp. Keep this run on the opposite side of the car from the power wire.
- Mount the amp. Fasten the amp to a solid surface, the enclosure itself or a board, with an inch or two of air around it so it can cool. Do not mount it upside down on carpet or bury it in trunk clutter.
- Connect the sub. Wire the amp’s speaker output to the subwoofer terminals on the enclosure, watching polarity. Positive to positive, negative to negative. If a dual voice coil sub, wire the coils to hit the impedance your amp wants.
- Land the ground and signal at the amp. Connect the ground, remote, RCA, and speaker wires to the amp. Double check everything is snug in the terminals with no stray strands touching a neighbor.
- Fuse the power at the battery. Mount the in-line fuse holder within about 12 inches of the positive battery terminal. This fuse protects the whole run if the wire ever shorts to the chassis. Connect the power wire to the amp first, then the ring terminal to the battery, then reconnect the negative terminal.
- Install the fuse and power up. Drop the fuse in last, turn the key, and turn the head unit on. The amp should light up. Now set the gain.
Tuning Basics
Wiring it is half the job. Tuning is what makes it sound good instead of just loud. Start with everything at minimum and bring it up.
- Gain. Gain is not a volume knob, it is a level match between the head unit and the amp. Turn the head unit to about three quarters volume, then slowly raise the amp gain until you hear distortion, then back it off until it is clean. Too high is the number one way people blow a sub.
- Low-pass filter (LPF). The sub should only play bass. Set the LPF somewhere around 70 to 100 Hz so the sub does not try to reproduce midrange. Adjust to taste and to how it blends with your door speakers.
- Bass boost. Leave it at or near zero. Bass boost is a fixed spike of gain at one frequency and it is the fastest way to overheat a voice coil. A little goes a very long way, and most builds sound better with none.
- Subsonic filter. On a ported box, set the subsonic filter just below the box tuning frequency so the sub is not wasting itself on frequencies the port cannot support.
Wire Gauge by RMS Power
Pick your power and ground gauge from the sub’s total RMS power. Use the same gauge for both, and step up a size if the wire run is long.
| Amp RMS Power | Recommended Wire Gauge |
|---|---|
| Up to 200 watts | 10 gauge |
| 200 to 500 watts | 8 gauge |
| 500 to 1000 watts | 4 gauge |
| 1000 to 1500 watts | 2 gauge |
| 1500 watts and up | 0 gauge (1/0) |
Common Mistakes and Safety
Almost every problem I get called to fix comes down to a handful of the same errors. Avoid these and your install will outlast the car.
- Bad ground. The single most common failure. Ground to sanded bare metal, keep it short, and make sure the bolt is tight to the chassis, not to a painted or plastic surface. A weak ground causes cutting out, whine, and weak output.
- Gain set too high. Setting gain by ear at full volume, or maxing it out, sends the amp a clipped signal that turns into heat at the voice coil. Clipped power kills more subs than too many clean watts ever will.
- No fuse or the wrong fuse. The in-line fuse near the battery is a safety device, not a suggestion. Without it, a chafed power wire against the chassis is a fire. Fit it within about 12 inches of the positive terminal and size it to the wire.
- Power and signal wires run together. Running the RCA next to the power wire injects engine noise into the signal. Keep them on opposite sides of the car.
- Reversed polarity. Wiring the sub out of phase makes it sound thin and lifeless because the sub and door speakers cancel each other. Positive to positive, negative to negative, every time.
Related Guides
- Best car amplifier for matching the right mono amp to your sub.
- Best under-seat subwoofer if trunk space is the problem.
- Best 12 inch subwoofers for the common all-rounder driver.
- Ported vs sealed car subwoofer box to choose the right enclosure.
FAQ
Do I need an amp to install a subwoofer?
For a passive sub, yes. A regular subwoofer needs an external amp to drive it, because a head unit cannot push the current a sub requires. The one exception is a powered subwoofer, which has the amp built into the enclosure, but you still run power, ground, and signal to it.
What wire gauge do I need?
Match the gauge to the amp’s total RMS power, and use the same size for power and ground. Roughly, under 200 watts uses 10 gauge, 200 to 500 watts uses 8 gauge, 500 to 1000 watts uses 4 gauge, and above that you move to 2 or 0 gauge. See the table above and step up a size for a long run.
How do I wire a sub to a factory stereo?
A factory head unit usually has no RCA preouts, so you use a line output converter. The LOC taps the factory speaker wires, typically the rear speakers, and converts that speaker-level signal into a clean RCA output you feed to the amp. From there the install is the same as any other.
Where should I ground the amp?
To bare chassis metal as close to the amp as possible, with the ground wire under about 18 inches. Sand the paint off down to shiny metal, bolt a ring terminal to it, and make sure it is tight. Never ground to a painted surface or a factory harness point.
Why is my sub not working?
Work through it in order. Check the fuse at the battery, check the remote turn-on wire is getting 12 volts with the key on, check the ground is on bare metal and tight, and check the RCA and speaker connections are landed and not shorting. A dead amp light usually points to power, ground, or the fuse. Sound but no bass usually points to signal or gain.
Sealed or ported box for a first install?
A sealed box is the easier, more forgiving choice for a first build. It is smaller, it is harder to get wrong, and it gives tight, accurate bass. Go ported when you specifically want more output and boom and you are willing to match the box tuning to the sub. The enclosure guide linked above walks through the trade-off.