Yep. Thats the slot. Sorry about no pics possible and I thought I could describe it. Then it just got more difficult to describe as I was typing... and you know how it goes.
Anyway, I tried it as shown above but it was just atrocious! I said to myself "There's NO WAY anyone design something so badly." I thought it must be a baby seat release or a release of some sort so I didn't push it too hard.
I worked back there installing my amp and running wires and never even noticed those slots. I had to go out to my car to see if I actually had them and I do. Tried pushing the seatbelt into it and it fit no problem. Works well but totally useless.