Adding a Subwoofer and Amplifier to a Honda Civic With the Factory Head Unit

Before touching any wiring, disconnect the negative battery terminal so the power feed can be handled safely. Plan the four runs that make up the install: a heavy power cable from the battery to the boot, a signal pair from the rear speakers to the amplifier via the LOC, a thin remote turn-on wire, and a ground strap in the boot. Keeping the power cable on one side of the car and the signal cable on the opposite side reduces the chance of alternator whine being picked up by the low-level signal.

Take off the rear door card, which is retained by three screws plus a row of trim clips, the same way the front cards come off. Lift the door-sill kick trim, which is held by clips, and peel back the rubber door seal so the centre trim section releases. This exposes the rear speaker connections and gives a clear path to feed cable between the door and the cabin. The rear speakers are the same physical size as the fronts on this car.

Take the amplifier's input signal from the rear speaker wires. At the loom each speaker uses two colours: on the rear left, positive is yellow and negative is brown, whereas on the rear right, positive runs blue with negative on orange. At the speaker itself the positive is the right-hand terminal when viewed from the back, on the blue cable. Rather than cutting the door wiring, a cleaner tap point is the multi-way loom plug behind the gear shifter in the centre console: fit spade or Scotchlok connectors there so the LOC feed can be removed later without leaving damaged wire. Split each tapped conductor so the original speaker still runs and a branch carries the same signal to the LOC.

Connect the tapped speaker pairs to the LOC inputs (R+, R-, L+, L-), and connect the converter's ground lead if your head unit uses a common ground. Using both left and right pairs gives an even signal and spreads the small load across both channels. Mount the LOC near the amplifier in the boot, extending the tapped speaker cable to reach it, and run RCA leads from the LOC outputs to the amplifier inputs. A basic converter is adequate for a bass-only setup; the one used in the original build cost about GBP 7.

The main power cable passes through the firewall at the rubber grommet in the centre rear of the engine bay, where the factory harness enters the cabin. Access is very tight, so removing the air filter box first creates working room. Feed the cable from the cabin side out into the engine bay rather than the other way: attach the end to a rigid wire or straightened coat hanger, ease the grommet back with a screwdriver, and pull the cable through. An alternative for large hands is to jack the car, remove the passenger front wheel and route the cable through the wheel arch, though this is no easier. Fit the inline fuse holder within a few inches of the battery positive before making the final battery connection.

Lift the door-sill trims and lay the power cable under the carpet along one side of the car, and the speaker/RCA signal cable under the carpet on the opposite side, both heading to the boot where the amplifier lives. Refit each trim panel as you go. Keeping the two cables separated helps avoid engine noise bleeding into the low-level signal.

The amplifier needs a remote lead that goes live only with the ignition so it powers up and down with the car. Three proven sources exist: tap the positive (yellow) wire at the rear 12V boot socket; tap the live wire behind the front cigarette-lighter socket; or, with the head unit pulled, splice into the antenna-amplifier power wire on the small black aerial connector. All three are ignition-switched, so the amplifier will not drain the battery when the car is off.

Fasten the amplifier ground to bare metal. Suitable points are the clip-in flap at the front edge of the spare-wheel well (file the paint back and bolt the ring terminal down) or the bolts on the handbrake-mount frame. A poor ground is the most common cause of weak or appalling audio from a stock radio, so scrape to clean metal and make the connection tight.

Land the power, ground, remote and RCA leads on the amplifier and mount it securely. If the amplifier has a built-in crossover, set it to low-pass for the subwoofer so only bass is passed; this is why only a LOC, and not a separate crossover, is needed on the signal side. Reconnect the battery, switch on the ignition and confirm the amplifier powers up. If it does not, briefly bridge the amplifier's +12V and remote terminals: if it then turns on, the fault is in the remote lead; if it stays dead, recheck the main power, fuse and ground. Set the gain to suit the head unit's output and test at moderate volume.