How do you install a rear facing infant car seat base in a 2001 Chevrolet Lumina?

I am trying to install a Graco car seat base in a 2001 Chevrolet Lumina. I am wanting to put it in the middle of the back seat, but there appears to be no latches. As a result, I was going to use the lap belt, but it doesn't seem to get tight enough. When it gets somewhat tight, it seems to easily loosen. Any help would be appreciated, as I'm not sure if I'm not seeing the latches or if they are just not in this model. Also, am I just not making the belt tight enough or doing something wrong? Thanks!