Do they make some cars where you can't get to the radiator directly and there's only a reservoir to fill?

I have a 99 Oldsmobile cutlass with a antifreeze leaking problem. I've only been putting the fluid in the reservoir because I can't find a direct radiator cap, it appears you can only put fluid in from the reservoir, is this possible in my car or is there always a way to directly put fluid into a car radiator?