Broken Antenna 96 Sable

Someone broke off the antenna on my '96 Sable. I bought a new one, but I can't figure out how to remove the pin that goes into the deck-mounted assembly. The round silver piece that sits on the rear quarter panels LOOKS like it should unscrew, because it has those four little slots, but when I try it feels like I'm going to break the whole thing.

Looks like I could remove a bolt on the inside of the trunk and remove the whole assembly, but I'd rather not, especially if this won't solve the problem.

Ideas?

Thanks,
Bill