Unfortunately, I have limited information on the boxes and peripherals you're using, but the AV port should work when used with the Wii and DVD player. Does just moving the switching box with the attached peripherals work? Or are you trying to use different inputs for each device?

Have you tried editing the name of the AV input with the device attached? Moving through the options in editing the name may bring the picture up on the screen, and gives options for different AV inputs.