A safer way is to use the car seat anchors that are in between the cushions in the back seat. All late model cars have these just for car seats. It's way safer than using a seat belt to hold the child's seat in. It's what I use and it's easy to install and remove the instead of having to fish the seat belt through the car seat. Pay close attention to the car seat installation instructions. So many people install the car seat incorrectly and find out too late that it was wrong.
As for your seat belt you can go to any junk yard and get one. They are held in by a Torx bolt size 50 if I remember right. Just fold the back seat down and you should have access to the seat belts and you'll also see those anchor hooks I was talking about above.
Let me know if you have any questions regarding child car seats as I have had 4 kids myself so I have plenty of experience with this [hihi] [thumb]