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
Multimeter
Wire Cutters
Pliers
Sharp knife or wire strippers
Torch
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