Don't! Most cars have a reservoir for radiator coolant ... add liquid to the reservoir. Opening the radiator cap introduces air into the sealed system ... you don't want air bubbles in the cooling system.
A lot of vehicles don't have the radiator cap on the radiator , they use a thread on style cap on the engine coolant reservoir ( that is the pressure cap / radiator cap )
Remove the radiator cap and add it directly to the radiator. Then fill the overflow reservoir to the full mark.
You add coolant to a C240 at the radiator overflow tank. This allows the radiator to suck in coolant as needed when hot.
To my knowledge, no car built on the CDW27 platform (Contours, Cougars, Mondeos, and Mystiques.) had a radiator plug, you add coolant directly to the coolant reservoir.
When your engine is cold add coolant to the coolant reservoir so it is up to the cold mark
Adding coolant to a 2003 Pontiac Grand Am is very easy to do. The coolant can be directly added to the radiator. Make sure the engine is not hot before adding the coolant.
On a 1996 Mercury Sable : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
Remove the radiator cap when the engine is cool and add the coolant into the radiator or you can add to the coolant overflow bottle. DON'T remove the radiator cap when the engine is hot!!!
Coolant reservoir, or directly in the radiator. If you are having to add much coolant, you need to find out why. Serious engine damage can occur if it is leaking inside the engine.
Radiator
no cap, just the coolant tank, add to this to add coolant../
Radiator. Remove the cap and add coolant there.