Picture of 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 :)

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?

ArniVidar3 years ago
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?
g199 (author)  ArniVidar3 years ago
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 :)
ArniVidar g1993 years ago
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.
g199 (author)  ArniVidar3 years ago
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 ;)
ArniVidar g1993 years ago
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 :)
g199 (author)  ArniVidar3 years ago
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
ArniVidar g1993 years ago
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. :)
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.)
g199 (author)  Yerboogieman3 years ago
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