Hi FightBack, I wanted to do the same thing myself a while back and after doing so I decided to make a few diagrams to help others though I can't remember if I ever got round to posting them.
Unfortunately somewhere along the way I've managed to loose my Advanced AV Cable so I'm unable to show pictures of the actual wiring which is a shame. Nevertheless I did sketch down the order of the wires when I was doing my conversion and will try to explain the process as best I can.
It is even possible to keep the SCART connection and be able to switch between it and Component HD.
As you have the SCART cable you have all the connections you need in the breakout box except the mode select wires.
Here is the pin definitions for the XBOX AVIP:
As you can see from the above table, pins 16 to 20 and pin 22 are common to both RGB SCART and YPrPb Component.
There are two ways you can proceed, either you Keep the SCART connectivity or you loose it.
The easier option is to loose the SCART connection.Without SCART
First open up the SCART plug and unsolder the wires to it. If you then strip back a few inches of the outer insulation to give you the individual coloured screened wires you can simply put RCA plugs on them for Y Pr Pb Left audio and Right audio. The remaining wires can then be trimmed back as they are not needed.
The screened wire functions within the SCART plug are as follows:
White wire = Left Audio
Red wire = Right Audio
Grey wire = Y Video
Blue wire = Pb Video
Yellow wire = Pr VideoWith SCART
If you choose to keep the SCART connection you will need to decide if you want to fit RCA sockets to the breakout box or to have then mounted externally on wires. You will need 5 in total (Y, Pr, Pb, Left audio, Right audio)
To connect them up I chose to solder my wires to the reverse of the circuit board under the 10 pin plug for video.
Here is a table with the functions of the wires going into the breakout box:
Using the table above and the one at the beginning of this post you can see that the grey screened wire is the Y video signal, the yellow screened wire is the Pr video signal, the blue screened wire is the Pb signal, the white screened wire is the Left audio signal and the red screened wire is the Right audio signal. All the other signals can be ignored.Video Mode Select
Finally you will need to change the video mode for your cable.
In case you didn't know, the xbox senses what type of AV cable is connected to it and changes the video encoder to suit. It does this by using 3 wires in the plug and depending on what combination of them is shorted to ground.
This can be seen in the table I created below: