If you have the top hat style rear disc rotors the parking brake pads are in the center. They should be horseshoe shaped in design. On most GM/Buick cars with this setup there is a star adjuster which can be adjusted to move the horseshoe pads out to compensate for wear or being uneven in application force. Each right and left rear wheels should have one. If this doesn't work then the horseshoe pads need replacement which is a job by itself. Plus they are not cheap. Also, you may have a turnbuckle style adjuster on the parking brake lines back towards the gas tank.
The other setup GM has is the rear disc caliper pistons are adjusted in or out to adjust the parking brakes.