Ok well first this is to fit rear speakers - then you have 2 options -
1 - run them from the headunit using the standard wiring, easy job just make sure the wires are in nice and securely
2 - Run an amp to drive all 4 'in car' speakers - little bit more work but the sound will be much better
With an external amp driving your speakers you will be able to get full power but this does involve buying an amp, power cable, earth cable and i would recommend getting some half decent speaker cable.
Im currently running some of this in my car -
http://www.chord.co.uk/products.php?pg=56 - and it sounds fantastic!