Yo,
Have you already got central locking (it's just not remote?) or have you not got central locking at all, and while you're at it you might aswell get remote?
I put remote/central locking in my mk3 1.4 and I have to say it was a hell of a lot easier than i thought it would be.
All you do is hide away the main box (which is only about 3" x 3") out of the way in the dash somewhere, run all of the solenoids to this box and using the link below simply push the connectors together.
At the door you screw a metal strip roughly where you need the solenoid to be, attach the solenoid to this strip and angle it so that the the metal wire screws against the locking mechanism itself and once it's locked it plunges the solenoid.
This then sends a message to the main central locking box (which you've hidden away) and tells it to lock/unlock all other doors. Repeat this process for all doors/boots. The only problem I had was attaching a solenoid to the boot as it doesn't work by metal wire and it's a solid kinda unit, so if you really want to do it you've need to drill into the little plate attached and angle the solenoid so that it triggers.
It probably took me about an hour to do both doors..
This link here, being as cheap as it is, may look abit shoddy/rubbish/cheap and nasty etc etc, but I have to say that this kit hasn't let me down, it's a very quick lock and unlock (unlike some of my friends mk3 central locking as standard) and if you read all the balls about why it's better than all the rest it's quite impressive..
http://cgi.ebay.co.uk/UNIVERSAL-REMOTE-KEYLESS-CENTRAL-DOOR-LOCKING-KIT-2008_W0QQitemZ330222276200QQihZ014QQcategoryZ75329QQssPageNameZWDVWQQrdZ1QQcmdZViewItemI'm babbling on about if you havn't actually got central locking in the first place btw, if not then you need to get something like
http://cgi.ebay.co.uk/Falcon-Phaser-Remote-Control-Central-Locking-Upgrade_W0QQitemZ280210714730QQihZ018QQcategoryZ75329QQssPageNameZWDVWQQrdZ1QQcmdZViewItemwhich I can't really vouch for...
Hope this helps