This gives quite a good description:
It seems pin 8 needs 6-12V but I don't think any of the other pins have
that sort of voltage level.
Whatever you, it might still involve some sort of manual intervention, so
you might as well select this input using the TV remote.
Oh well. I actually run the Wii through a Humax PVR and switching sources
requires me to turn the blasted thing on which is why I want to do this but
it seems that either "12V + button" or " live with it" are the ways to go