You will need a line out converter, amplifier, amplifier wiring kit, subwoofer, and a sub box.
You will need to tap into the wires in your headunit wiring harness, for the LOC, then run a power cable from the battery to your amp This will require you to connect it in the engine bay, and drill a hole through your firewall to snake it through. Run the cable under the carpet, to the location of your amp. If you dont plan on changing out the speakers and powering them, then i recommend putting the amp in the back of the car near the sub. Run a ground from the amp to the closest point on the car. Try to stay less than 6 feet.
From the LOC, run your RCA Cable under the carpet back to the amp. Make sure that they stay well clear of the power cable you just ran, if you can, run them down the opposite side of the car.
tap into a power cable from the headunit down the side of the car with the power cable to the amp. This is the amp remote cable, it will turn the amp on and off as the headunit turns on and off, so make sure that you tap into the right cable.
Finally, install your sub in your box, and run speaker wire from your box to your amp.
In truth though, you will find that getting a new headunit will make this job alot easier, and in the end, will give you much better sound quality. This is also a bit of a project, with intermediate wiring/soldering required. If you are not 100% sure you can handle it, your best bet is to take it to a shop.
i recommend using a new sub wiring kit, they are fairly cheap, and its better to get new ones than take the chance that yours are bad/too short/etc.
Your hardest job is going to be hooking up the wiring to the headunit. It will require doing research to find out which wires are which, and then stripping them, soldering on your LOC and Amp Remote wire, and then running them to the amp. Im not sure how easy it is to run a cable from the engine department back on the euro models, you have to drill through the firewall at some point, and that could be easy or hard depending on the layout of the euro models.