Rich,
As I wanted the multimedia box to be powered only when the MFD2 was switched on (no point otherwise), I wired the permant Live to the CD changer socket (on the back of the MFD2) 12V out, so that only when this pin saw power did the unit power up. The earth was taken from the main MFD2 earth coming in, and the final 12V for video in motion (VIM) was taken from the cigarette lighter. I fitted an override micro switch inside the ashtray, and the lighter feed was the closest. The swtich was very convenient for switching the VIM on or off.
In fact the switch is still fitted (see below).
Car looks stock and unless you knew where the switch was, you would never spot it.
Edit: I've just checked and the CD changer power was Terminal 4 of Connector 6 (CD Changer) - "CD changer, voltage supply, positive, terminal 30"