This happened to me on a 2002 Cadillac Seville. I think the rear calipers are the same. You have to disconnect the cable from the caliper. I did this by removing the cable bracket that attaches to the caliper. Then pulled the cable end and got it disconnected. Move the parking brake mechanism on the caliper and see if it returns freely. If it does the caliper is just fine. If not you need to replace one or both calipers. On the Caddy it was the rear parking brake cables. The Caddy had 4 separate cables not sure about the Grand Prix. The cables were visually fine but were destroyed internally.
Don
