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Not necessary on this vehicle.
You should not need to bleed coolant. Ever. If the coolant is filling up on its own the stuff in there is not coolant and you have a big problem.
The coolant air bleed valve is located on the top of the thermostat bypass pipe/heater pipe assembly. Close the valve once a continuous stream of coolant is expelled from the valve.
Cold engine. Remove the radiator cap, top it off, start the engine, add coolant as necessary until all air bubbles stop escaping.
Service the cooling system. Drain the coolant and replace the thermostat which is more than likely stuck open. Pour in a 50/50 mix of antifreeze and distilled water. Bleed all air from the system.
It's not easy
This should not be necessary on this vehicle.
I don't think that this problem is unique for a vehicle of it's age. You probably either have some blockage in the heater core or your coolant system has a bad thermostat or the coolant system has a significant air pocket (that needs to be bled out.) The cheapest thing is to try and bleed the system, then replace the thermostat, then flush the coolant system. In a rarer situation you may have a vacuum leak that effects the valve controlling the circulation of water into the heater core. None of these are expensive but are time consuming.
Air Conditioning Vacuum Pump.
Air trapped in the system. Bleed the brakes.
The radiator on a 1999 Pontiac Grand Prix is flushed by draining the coolant, filling the system with water, adding a liquid flush to the system, and running it for a few minutes. The system is then drained, flushed with fresh water, and refilled with coolant.
You're simply going to have to have it removed and replace it. That nipple is at the lowest point in the system and there is no other way to drain it unless you have the system power flushed.