The heater inlet hose on a 2000 Pontiac Grand Prix GT is located on the
engine side of the firewall, near the
heater core. It's a relatively thick hose and will be connected to the
heater core inlet. You'll likely need to look closely in the engine compartment, possibly using a flashlight, to locate it. It will be one of the hoses going through the firewall. The precise location can vary slightly depending on engine type (3.1L or 3.8L), but it's always in that general area.
To be certain, you may want to consult a repair manual specific to your vehicle's year and engine. A diagram would provide the most accurate location.
