You can't see the notch unless the rear seat is out and the 6 x 9 just the middle edge slides partially into the notch so it can sit flush with the deck.I guess where the notch is in relation to the seat I guess. I'm trying to picture it, but there usually isn't much excess room on that deck.
I am sure if you lay in your trunk and stick the 6 x 9 up to the deck you will see what it needs to fit, that is how I figured I could do it after buying adapters that made my magnets hit my trunk bars.