Sounds like you bought the x2c motorsports kit. I had the same problem. No hardware, no instructions. I returned it for a kit from the Yellow Mustang Registry. Their kit was awesome, straight forward instructions and hinged so you can store stuff under it.
I would not used the rear seat delete to mount my child's car seat though. They aren't designed to take much weight. The most I've seen mounted on them is a NOS bottle.