Adding RCAs to a Standard Car Head Unit





Introduction: Adding RCAs to a Standard Car Head Unit

If you want to upgrade your sound system in your car by adding a sub woofer or speakers powered by an auxiliary amplifier without having to get a new head unit then hopefully this should help you :)

I decided to keep my standard head unit because it looks well in the car, is less likely to attract thieves than a shiny after-market unit and it is a factory upgraded unit anyway and i find it gives pretty good audio quality, it works with the steering wheel controls and has bluetooth and aux-in, so an after-market unit wouldn't offer a substantial upgrade.

Factory head units rarely have RCA or "Low line" outputs which your amplifier will need for it's input signal.
A "Low line converter" is a device which will piggy back your speaker outputs from your head unit and convert them to Low Line RCA outputs for your amplifier

The converter i bought also gives you a "remote wire" output which tells your Amp when the car is on so it doesn't drain your battery, if you get a converter without this option you can just join the remote wire to the Acc circuit of your car, or the 12V circuit with a switch (you will have to remember to turn it off though or your amp will drain your battery)

The methods i used will require an understanding of car/car stereo electrics so just be careful so as not to damage your car or yourself :)

Step 1: Bits and Tools

Bits You'll need:

Low Line Converter, I got mine on Ebay here for £6.00
Stereo Keys to remove your head unit, you may not need these depending on the manufacturer and fitting type in your car
Wire Cutters
Sharp knife
or wire strippers

Electrical tape or heatshrink tubing
Wire to extend the piggyback cables on the converter (i used around 20 AWG)
Soldering iron to solder extension wire if needed
Scotch Blocks i ended up using 6
Cable ties not essential but handy for keeping wiring neat

Before you unplug your factory fitted radio, make sure you have the "unlock code" usually in the owners handbook. If you dont have it then you should acquire it from your car manufacturers customer service phone number, your local dealer or the internet.
Getting them online can be a bit unreliable and expensive. I already had mine but i certainly wouldnt pay more than £5 for the dealer to look it up on a database. If you dont have the code and you unplug your stereo it will not allow you to listen to it as it will assume it's been stolen. Don't guess the code as you usually only get a few attempts before it will permanently lock it'self!

Step 2: Wiring Plug

Remove your head unit. At the back of your stereo there is most likely one big plug which incorporates all of the inputs and outputs to and from your head unit.

You will need to find a wiring diagram for your plug type or work out which wires are which
You can find most wiring diagrams on-line, most Ford plug diagrams are available here
Also if your local dealer is nice they may give you a hand, though be sure to look on the internet first as they'll probably want money

Be aware that the diagrams are usually of the Pins and they are normally in the head unit, the "plug" is made up of all the sockets which correspond to the pins, so you will have to treat the plug as a mirror image of your diagram, but check your diagram is correct with a multi meter anyway to  be sure

Test your diagram with the below:

The resistance between + and - speaker channels (on the same speaker) should get around 4 Ohms

Check for 12Volts between the 12v pin and the ground pin or the car chassis

You will also need an "Acc 12V" or "Accessories 12V" which will only be 12V when the key is turned the first click in the ignition, and will read below 1V when the key is not turned, that will also be between the Acc pin and ground

Step 3: Mount Your Converter

Actually, don't mount it yet.

Find where you are going to mount it and work out if you need to add any wire to your converter to get it to reach comfortably to where your head unit cables are. Take into account that your head unit will need space behind it to fit back into the dash properly so don't count on splicing into the cables right at the plug with the converter right behind the head unit, it may be better to mount it somewhere else and lengthen the wire to reach.

I initially thought that behind the glove box would be the best place but i ended up cable tying it to a heater pipe that runs down the centre console.

When you have a rough idea where you will be mounting your converter unit, solder on the extra wire you'll need to reach your head unit cables and insulate them with electricians tape or heat shrink tubing

Step 4: Splicing In

Okay, now you'll need to match up your head unit wiring with the wiring required by your converter.
Your converter should come with wiring instructions so you will just need to match up the wires and splice them with the scotch blocks, use the pliers to press the metal splice bar so you can be sure of a good connection.

When you have done it is a good idea to check speaker splices by doing a continuity test between the plug pin and the new wire
And 12V, Acc 12V and grounds between each other using your new wire and the plugs pins using voltage tests to prove the connection is good (or not, but i had a 100% success rate with the scotch blocks)

You can Solder the wires if you don't want to use scotch blocks BUT i would advise against it, the scotch blocks are REALLY easy to use, you don't have to worry about burning your interior or cold solder joints which will be a pain later on

Image 2 shows how the blocks should look, the "stereo wire" would be the long one which comes out both sides (the one with the black test lead on it)

Image 3 shows how the block had spliced into the two wires and joined them electrically

Step 5: Test

Okay so now you're converter should be working :) yay!
Mine only comes on when the Acc +12V circuit is activated with a click on the key turning in the ignition, at this point a blue light on the converter comes on, but that will be dependant on your converter and how you wired it, although i'm sure the converter draws a minuscule amount of power it's probably best not to have it "on" all the time, so be sure it is off when your car is

Plug in your RCA plugs and check that the signal is being taken to your Amp or Sub, wherever the other end of the RCA wires are
If it works, check the remote wire output from your converter by using a voltage test between the wire and a ground, you should get a + reading from 5V to 12V depending on your converter.

Have a cup of tea while you warm up (it was -2 Celsius all day when i did this!)

Tape up your scotch blocks to stop them rattling around and to keep the cables bundled up neatly. Then unplug everything, put it all back in your dash and plug it all back together again

If it doesn't work, make sure you have tested the connections with a multimeter and redo any that fail
Also, my converter has "Gain adjusters" on the PCB you can adjust with a screwdriver, be sure they aren't turned all the way down, i'm sure they don't go all the way to 0 signal but it's best to be sure :)

Step 6: Right Then....

Everythings working, you just need to mount your converter to something, only really to stop it rattling and tapping when you're driving, then cable tie any loose wires together and make sure they won't interfere with things like your glove box lid. Avoid cable tiying the extension wire to solid things, as it will make removing your head unit difficult in future. Otherwise you can just undo the plug from the converter and then the back of the head unit.

Run your RCA wires and remote wire to your Amplifier and then enjoy improved music quality :)

Hope i have helped, Please comment



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    Very cool, but one contemplation here.

    You are basically parallel-connecting the 4ohm speakers with the Low-Line converter. Assuming that the converter's inputs are 4ohm themselves, that leaves 2ohms of resistance for the stereo system to power to. Not all stereo systems can power to 2ohms without simply burning over the circuits.

    Have you encountered any issues with your system after doing this?

    11 replies

    I need some advice. I keep getting a humming sound from my subwoofer. I have used a speaker 2 rca with auto remote on a 4 guage wire kit. Any ideas on how i could stop this noise??

    Did you run RCA cables and power wire down the same side of the car? Try running power down one side and RCA the other. Make sure power wire is same gauge as ground off the amp.


    HI i have the same with my sub at home. its the poer going in is to high for the amp and results in a humming sound for the extra power. try turning the AMP down and your stereo up


    I've had no issues at all, the converter doesn't put a load worth considering on the rear speaker circuits as it's only really "listening" to the signal, it draws all its power separately from a 12v feed. so it's not like adding an extra speaker to the circuit. hope this helps :)

    Very cool, thank you.

    I have a low-level converter here (not that exact brand you're using, but surely they must all behave the same way) that I'm going to be installing in my car, but I didn't want to lose the 4 in-car speakers (regardless of how weak and pathetic they are) in the process.


    I would expect that they're all based around the same circuitry regardless of brand. all of my original speakers work still, whether the sub is on or not. As i say its not really influencing the signal to the rear speakers, i doubt it makes any difference you could detect with your ears even at high volume. Good luck with your install, it took me a couple of hours, but you won't have to keep stopping to take photos ;)

    Hmm.. previous comment disappeared :S Take two:

    Thank you :)

    I may not need to stop to take pictures of the process, but in addition to the RCA outputs I also need to create and install a 12V->5V converter, a 7-port powered USB hub, headphone->radio transmitter, and other assorted goodies. All that stuff plus the amplifier, power capacitor, noise filter and extra speakers will probably take me YEARS :)


    A power cap? how big an amp are you running? if you're venturing that far into car audio it's probably going to be worth a new headunit

    Well, it's not so much that the amp is huge (it's only 4x100W RMS), but that the Yaris' battery is TINY. I'm thinking that a small-ish power capacitor (maybe 0.5F or so) would help with the spikes and not render the poor battery empty with every kick of the bass :D

    You do realise that it's the battery that charges the capacitor, right? ;)

    Yes? Do you realize what the power capacitor does in a car stereo system? It helps the system cope with the spikes in power required to punch the bass, by reducing the load on the alternator and car battery. :)

    Seems you know your stuff maybe you can help me out with this problem???

    Will it work in reverse, I have a new head unit going to a 97 SC400 OEM Nakamichi Amp? There is only front and rear speaker input but the amp also supplies a woofer? If not what would I need to get these two systems working, was also hoping to get the 12 Disk CD changer connected up -- trying to keep the system with original OEM harness as much as possible..easy or a challenge?

    Thanks in advance if you can help


    WireHarnessLexus121001 Stock Stereo Think its Nak 97 or earlier.jpgLexus_SC400_Radio_NAK_Wiring_Diagram_Page_1.jpgLexus_SC400_Radio_NAK_Wiring_Diagram_Page_2.jpgLexus_SC400_Radio_NAK_Wiring_Diagram_Page_3.jpg

    This is the 1 i used.


    I have connect a speaker to rca with remote turn on/off. Everything works fine but sometimes i get a loud humming sound from the subwoofer. This happens when i switch the car off or when i open the door. Any ideas on how i can prevent this from happening??

    and the REM wire is which one? i was thinking of hooking it up to the ignition wire so it is turned on and off with the turn of the key, any problems with this?

    I have installed a kit like you are talking about, it was 4 or 5 speaker wires that were spliced total. For the REM wire, I recommend wiring it to a fuse in your fuse box to prevent battery drain. Make sure the fuse you choose is only active when the car is on.

    How many wires needed to be spliced in the end? And I'm having trouble getting my power wire from the battery in my mk6 fiesta to the boot, any ideas?

    I thought about this but was worried about losing sound quality and I wasn't sure how well it would work with my 5-channel amp. (Besides, the stock radio didn't have as many options as yours.)

    1 reply

    I was really surprised at how good the sound quality is, I only run a sub through it but I tested some speakers and I couldn't pick up any signal loss, if the radio isn't as complicated it should only really make it easier to work out which wires are which, but obviously don't chop into them if you're not sure. Thanks for the comment