two of the hoses going into the firewall are the two sides of the heater core. check coolant level to make sure you don't leak when you unhook them. take them both off and spray a garden hose into one side. If you can manage to hold a bucket or tub under the other side, that helps keep water from getting on stuff. make sure you cover your electronics and stuff, i haven't done it on a grand prix but this fixed the issue with my old blazer