I searched the owner's manual and there was no mention of folding the rear seats down. If you don't have a manual, you can get one here: http://www.extendedgmwarranty.com/owners-manual/buick/1999-Buick-Century.pdf
You can remove the seats following the instructions below from autozone.com.
Fig. Typical W-Body rear seat cushion removal
- Locate and remove the small bolts at the front lower edge of the seat cushion.
- Pull the tabs on the rear cushion retainers.
- Remove the seat cushion by lifting up and pulling out of the retainer.
- Installation is the reverse of the removal procedure. Make sure the retainer tabs are snapped into place. Tighten the bolts to 18 ft. lbs. (25 Nm).
Fig. Typical W-Body rear seat back removal
- Remove the seat cushion as described above.
- Remove the anchor nuts securing the rear seat retainer at the bottom of the seat back.
- Grasp the bottom of the seat back and swing upward to disengage the offsets on the upper frame bar from the hangers.
- Lift the seat back upwards to remove from the vehicle.
- Installation is the reverse of the removal process. Make sure the hooks on the seat back are properly installed to the hangers. Tighten the anchor nuts to 89 inch lbs. (10 Nm).