The back of the headliner is sort of wedged against a piece of steel. Its kind of hard to describe but you can pull it down just enough to get your hand up in there and take undo the bolt for the antenna base. It'll probably put you in an awkward position against your rear windshield but you can do it. I know because i did this to replace my antenna with a sharkfin one. Be careful not to bend your headliner too much or else you'll leave a line in it. I'll see if I can find a picture to help and perhaps figure out a better way to describe how its attached.
EDIT: Excuse my finger but this is from my interior conversion thread, you can see the lip on the back of the headliner there.
