Where is the radiator cap on a 2000 Saturn sl2?

Your car does not have a radiator cap. You can only add coolant through the overflow reservoir. It takes a while. You have to fill it, start the engine and run it until it drinks it in, shut it off, fill it again, repeat repeat repeat. As a general rule, if you get to the point where you can have it running for 10 min without drinking any in, and your heater blows hot air, you are done.