How To Tap Into Rear Speakers For Subs?

Want to get more bass in your car without running new wires? You can often use your existing rear speaker wires to connect a subwoofer. This guide will show you how to tap into rear speakers for subs, a common method that can save you time and effort. It’s a practical way to enhance your audio experience with deeper, richer bass.

This approach is especially useful for simpler setups. It lets you add a subwoofer by using the signals already going to your back speakers. We’ll walk you through everything you need to know, from the tools required to the step-by-step installation.

This method involves connecting your subwoofer amplifier to the audio signal sent to your rear deck or door speakers. Your factory radio or aftermarket head unit sends a full-range signal to these speakers. By tapping into these wires, you can send that signal to an amplifier, which then powers your subwoofer.

It’s a cost-effective technique. You avoid the need for a new head unit with dedicated subwoofer outputs. However, it’s important to understand both the advantages and the limitations before you start.

Why Use the Rear Speaker Wires for a Sub?

There are several good reasons to choose this method. It’s often the simplest path to better bass.

No RCA Outputs Needed: Many factory stereos lack RCA pre-amp outputs. Tapping into speaker wires bypasses this requirement.
Minimal Disassembly: You usually only need to access the rear speaker areas, not the entire dashboard to get to the head unit.
Cost-Effective: It can save you money on adapters or a new radio, especially if your current system otherwise meets your needs.
Integrated Fader Control: Your head unit’s fader control can sometimes be used to adjust the subwoofer level relative to the front speakers.

Potential Drawbacks to Consider

While useful, this approach isn’t perfect for every situation. Being aware of the downsides helps you set realistic expectations.

Signal Quality: You are using a high-level speaker signal, not a clean low-voltage pre-amp signal. This can sometimes introduce more noise.
Bass Roll-Off: Many factory radios apply bass roll-off (reduction) at higher volumes to protect the small factory speakers. Your subwoofer amp will get this rolled-off signal, limiting its output.
Limited Control: You might not have a dedicated subwoofer volume control from the driver’s seat unless you add an in-line controller.

Tools and Materials You’ll Need

Gathering the right gear before you start makes the job smoother. Here’s a basic list to get you going.

Car audio amplifier (mono or multi-channel bridged for a sub)
Subwoofer in an enclosure
Amplifier wiring kit (power cable, ground cable, remote turn-on wire, fuse holder)
Wire strippers and cutters
Crimping tool and connectors
Line Output Converter (LOC) – This is crucial for most installations
Electrical tape and/or heat shrink tubing
Basic hand tools (screwdrivers, socket set for battery terminal)
Multimeter (helpful for identifying wires)

The Critical Role of the Line Output Converter (LOC)

The LOC is the key component in this process. You cannot connect speaker wires directly to your amplifier’s RCA inputs. Speaker wires carry a high-power, high-level signal.

The LOC converts this powerful speaker-level signal down to a low-level RCA signal that your amplifier can safely accept. It also balances the signal, which helps reduce noise. Some LOCs are basic, while others offer gain adjustment and bass restoration features to combat factory roll-off.

Step-by-Step Installation Guide

Follow these steps carefully to ensure a safe and functional installation. Always disconnect the negative terminal of your car battery before working on any wiring.

Step 1: Plan Your Layout

Decide where you will mount the amplifier and place the subwoofer box. Consider airflow around the amp and secure mounting for safety. Plan the route for the power cable from the battery through the firewall to the amp.

Step 2: Run the Power Cable

Disconnect the negative battery terminal.
Connect the fused power cable to the positive battery terminal.
Find a suitable grommet in the firewall to pass the cable into the passenger compartment. Avoid sharp edges.
Run the cable under trim panels along the side of the car to the amplifier location.

Step 3: Locate and Access the Rear Speaker Wires

You’ll need to find the wires going to your rear speakers. Common access points include the rear deck panel, door sills, or directly at the speakers themselves. Consult a vehicle-specific forum or wiring diagram for the best location and wire colors.

Step 4: Identify and Tap the Correct Wires

Use a multimeter set to AC voltage to confirm the speaker wires. Play a test tone or music at moderate volume – you should see a changing voltage.
Identify the positive (+) and negative (-) wires for both the left and right rear speakers. Usually, the wire with a stripe is negative.
Using your wire cutters, strip a small section of insulation from each of the four wires (Left+, Left-, Right+, Right-).

Step 5: Connect the Line Output Converter

This is the core of the tap. Connect the speaker wires to the high-level input side of your LOC. Follow its instructions precisely.

Connect Left Rear+ to the LOC’s Left+ input.
Connect Left Rear- to the LOC’s Left- input.
Connect Right Rear+ to the LOC’s Right+ input.
Connect Right Rear- to the LOC’s Right- input.

Secure the connections with crimp connectors or solder, and insulate them well. Mount the LOC securely nearby.

Step 6: Run RCA Cables and Remote Wire

From the LOC, run your RCA cables to the amplifier’s input. Also, run the remote turn-on wire. You can often tap this from a switched power source in the fuse box (like the radio fuse) or from the LOC if it provides a remote output.

Step 7: Connect and Ground the Amplifier

Connect the power cable to the amplifier’s +12V terminal.
Connect the remote turn-on wire to the amp’s “REM” terminal.
Find a solid, bare metal point on the car’s chassis near the amplifier. Sand away any paint and attach the ground cable securely here. Connect the other end to the amp’s GND terminal. A poor ground is a common source of problems.
Connect the RCA cables from the LOC to the amp’s inputs.

Step 8: Connect the Subwoofer and Final Check

Connect the subwoofer to the amplifier’s speaker outputs, observing the correct polarity (+ to +, – to -). Double-check all connections, especially power and ground. Reconnect the car battery negative terminal.

Setting Up and Tuning Your System

With everything connected, it’s time to configure the amplifier for the best sound. Incorrect settings can lead to poor performance or even damage.

Initial Amplifier Settings

Start with conservative settings on your amp before playing any music:

Gain/Level: Set this to minimum.
Low-Pass Filter (LPF): Turn this on and set it to around 80 Hz. This ensures only bass frequencies go to the sub.
Bass Boost: Turn it OFF to start.

How to Set the Gain Properly

Play music you are familiar with at your typical listening volume.
Slowly increase the gain/level control on the amplifier until you hear the subwoofer distorting or the sound becomes “clipped.”
Once you hear distortion, back the gain down slightly until the sound is clean again. This is you’re maximum safe level.

Troubleshooting Common Issues

If things don’t work right away, don’t worry. Here are solutions to frequent problems.

No Sound from the Subwoofer

Check the fuse at the battery.
Verify the ground connection is solid and metal-to-metal.
Confirm the remote turn-on wire has 12V when the radio is on.
Ensure the LOC is connected to the correct speaker wires.

Engine Whining Noise

A whine that changes with engine RPM is usually a ground loop. Ensure the amplifier ground is short, secure, and to bare metal. Try grounding the LOC to the same point as the amplifier. Keep RCA cables away from power wires.

Weak or Distorted Bass

This could be bass roll-off from the factory radio. Consider upgrading to a LOC with bass restoration circuitry. Also, re-check your gain setting to ensure it’s not too low or too high.

FAQ Section

Can I tap into front speakers for a subwoofer?

Yes, you can tap into front speakers instead. The process is identical. Some prefer the front channels because they often carry a fuller range signal, but using the rear is common and effective.

Do I always need a line output converter?

Most amplifiers require one. However, some amplifiers have “high-level input” terminals that accept speaker wires directly, eliminating the need for a separate LOC. Check your amplifier’s manual.

Will this hurt my factory radio?

No, when done correctly, it places no extra load on the factory radio. The LOC has a high-impedance input that only “listens” to the signal without drawing significant power.

How can I control the subwoofer volume separately?

You can purchase a LOC with a remote level knob, or you can install an in-line RCA level controller. This allows you to adjust the bass from the driver’s seat.

Is this better than using RCA outputs from a new headunit?

Using dedicated RCA pre-amp outputs from a good aftermarket head unit usually provides a cleaner, stronger signal with more control. But tapping rear speakers is a great, less expensive solution for adding a sub to a factory system.

Adding a subwoofer by tapping into your rear speakers is a very achievable project. It can dramatically improve your car’s audio without a complete overhaul. By following these steps, using the right converter, and taking your time with the setup, you’ll be able to enjoy that deep, powerful bass that makes music and movies come alive. Remember to prioritize clean connections and safe wiring practices throughout the process. With a little patience, you’ll have a much better sounding system.