if i remember correctly it is PIN 8 of your scart connector, you need to supply some voltage to it, in order for your tv to always power on in AV mode. you need to supply at least 4.5 volts, or up to 12 volts -- note: your screen may switch between widescreen, and normal 4:3, depending on the amount of volts you supply to pin 8.
so eg. if 5-7volts causes your tv to go to widescreen, try 10-12volts instead for 4:3...