I already know that I am an idiot. Where is the radiator cap on a 1999 Olds Alero?
The cap is not always on the radiator itself. Follow the upper radiator hose toward the engine. Any pressure cap you find (“Do not open when hot”) is the one you want.
The cooling system pressure cap is located on the coolant reservior.
As you stand at the front facing the engine compartment, you will see an off white plastic container to your left on the inner side of the fender. (That’s the right side of the car.) The cap is on top of that reservoir.