Hi me_ashman,
Didnt need to enter any codes.
Are you ordering the screen from china(Jessica) or ebay?
What device are you going to use for audio input?
You can route the VGA & USB cables behind the back of map compartment along, I've also connected the three video inputs (rear camera in the future).
The hard bit is actually pulling the cup holder out, took me several attempts before I finally got the bugger.
There are some examples about removing the HU with images, that was my initial reference point, have a search on the mazda forums.
By the way, this is my next purchase
In Dash PC - GROM PC adapter for Mazda 02+, its basically a replacement for the ipod autolink that I'm using for my audio input, the car2pc allows you to receive the commands from the steering wheel to the PC, so in my case Roadrunner will change tracks in windows, etc. without me touching the screen.
Its taken me a few months to get everything setup, I would recommend a M2-ATX or M3-ATX psu power supply (or use a laptop and build a computer later)