Bits You'll need:
Low Line Converter,
I got mine on Ebay here
to remove your head unit, you may not need these depending on the manufacturer and fitting type in your car
or wire strippers
or heatshrink tubing
to extend the piggyback cables on the converter (i used around 20 AWG)
to solder extension wire if needed
i ended up using 6
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!