Upgrading your car's factory stereo system can take your music listening experience from dull and lifeless to vibrant and immersive. However, rewiring the system usually requires splicing into the original harness which can be complicated and risky for your car's delicate electrical system. Thankfully, there are methods to dramatically improve sound quality by tapping into the rear speaker wires alone, leaving the rest of the wiring untouched.
Assessing Your Factory System
The first step is understanding what you're working with. Pop off the factory stereo and look at the rear of the unit. Note the wire colors and where they connect to the harness. The key ones we care about are:
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Power (constant 12V red) - Provides power to the stereo at all times.
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Ground (black) - Provides the reference 0V ground.
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Accessory (blue/white) - Turns on the stereo when the key is in the accessory position.
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Ignition (yellow) - Turns on the stereo when the key is in the on position.
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Dimmer (orange/white) - Adjusts display brightness based on headlights.
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Antenna (blue) - Provides antenna signal to the tuner.
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Speakers (gray, gray/black, white, white/black) - Send amplified signal to each speaker.
We will tap into the rear speaker wires to insert our own amplifier, leaving everything else untouched.
Shopping for New Gear
To make this work, we need two new components - an amplifier and a line output converter (LOC).
Amplifier
This will provide amplified signal to our new speakers. Look for an amp between 50-100 watts per channel to sufficiently power most aftermarket speakers. Ensure it has low-level RCA inputs. Major brands like Alpine, Rockford Fosgate and JL audio make excellent amplifiers in this range.
Line Output Converter
The LOC will convert the rear speaker output from the factory stereo into an RCA line-level signal we can feed into the amplifier. There are two types:
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Passive - Simple resistor-based networks costing $15-30.
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Active - Using op-amps and buffer circuits costing $50-100.
Active LOCs are higher quality and recommended if you want the best performance. Brands like AudioControl and PAC make great active LOCs.
Running New Wires
We'll need to run power and signal wires for our new amp.
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Use 12 or 14 gauge wire for power from the battery. Add an in-line fuse holder near the battery end.
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For signal, run 16 or 18 gauge wire from the LOC output to the amp input.
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Use quality copper wire like KnuKonceptz Kolossus Fleks.
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Secure and hide wires by running under trim panels and taping up neatly.
Connecting the Gear
With all the gear and new wiring in place, we can now make our connections:
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Unplug the factory stereo and tap the rear speaker outputs with the LOC input wires.
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Route the LOC RCA outputs to the amplifier location.
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Connect the amplifier power wire to the battery and ground to chassis.
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Plug the LOC RCA output into the amp input.
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Connect the amplifier speaker outputs to your new speakers.
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Reinstall the factory head unit.
Once everything is securely connected, test it out! You should now have a loud, clean signal going to your new speakers without ever touching the factory wiring.
Tuning Tips
To get the most out of your new sound system:
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Adjust amp gain to match stereo output voltage. Overdriving the amp causes distortion.
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Balance fader so front/rear speakers play at same volume.
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EQ boost bass and treble range for more impactful sound.
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Add sound damping inside doors to reduce panel vibration.
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Upgrade front speakers to match the rears for full 360° sound.
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Add a subwoofer for booming bass by using a line-out converter on the rear speaker wires or tapping an unused pin on the harness.
With a little wiring know-how, you can dramatically enhance your car's audio system while keeping the factory setup intact. Follow these steps and you'll be enjoying supreme sound quality in no time.