You fill it though the resivoir cap
The radiator does not have a cap, instead, the coolant system reservoir is also pressurized and the pressure cap is on the reservoir.
The radiator cap is on the coolant reservoir, which is mounted on the passenger side fender for S series Saturns.
It's only radiator cap is on the coolant reservoir.
The 2001 Windstar has no radiator cap. It has an antifreeze bottle.
On a 2001 Lincoln Navigator : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
On a 2001 Ford Expedition : The thread on cap on your engine coolant reservoir is the pressure cap / radiator cap
On a 2001 Ford Focus : The thread on cap on your engine coolant reservoir is the pressure cap / radiator cap
The thread on cap on your engine coolant reservoir is your pressure cap / radiator cap
On a 2001 Ford Expedition : The thread on cap on your engine coolant reservoir is the ( 16 PSI ) pressue cap / radiator cap
On a 2001 Ford E-150 : The thread on cap on the engine coolant reservoir is your pressure cap / radiator cap
If Im not mistaken, the Pressure cap for a Saturn is the twist on cap on top of the coolant resovoir, if the cap say under pressure then that is it and to add coolant to radiator you fill it thru the res.