How do I put back seat of 2004 Impala back in?

I had to take my back seat out. I was trying to find some wiring that I knew was back there. The bottom of the back seat popped right out with no screws or anything like that. I went to put it back in and I can't get the back seat to go in anymore. On the bottom of the back seat, there's a metal square loop kind of a thing. That goes into a plastic piece. When pulling out the seat, the plastic was still around the square metal loop. And then the plastic piece goes into a squarish hole in the sheet metal where the seat sits on. I can push the seat back down into the sheetmetal's hole. But then with the lightest pull up of the seat, it pops right back out of place. How is this supposed to go together? I tried to take the plastic piece off of the metal square loop but I couldn't get it to come off. I was hoping if I got that off, I could put the plastic piece into the hole in the sheet metal and get it in there good and then the seat would just fall into place into the plastic piece and be snug. Or does the seat pull out that easy all the time? I never really tried pulling up on the seat before today. Maybe it always pulls out easy.