The Wiki page you linked to tells you all you need to know. ~12V on pin 8 to switch your TV to SCART input, ~6V to both switch to SCART and widescreen (if supported by TV). Pin 21 is ground for this input.

In your case I think you will have to reassert this input after you've switched your TV on, or otherwise it will remain on whatever channel you had to use to switch it on.