I believe that GM started including tether anchors in MY 2000 vehicles, so yours likely doesn't have one. You can have them install 1 for free (per service bulletin no. 99-09-40-004) or you can do it yourself.
Check on your back parcel shelf. Behind the center of each seat, there should be some kind of indentation on the shelf that is the hole for the anchor. Having side speakers may move the anchor locations to the inside slightly. You should also be able to see the welded nuts on the underside of the shelf when you look in the trunk.
If your dealer gives you a hard time about installing the anchor, just give Pontiac a call at 800-762-2737 and let them know the situation.
Good luck!