From my 2010 Flex service manual:
1. remove the L & R "D" pillar trim panels
2. Remove rear dome lamp
3 Lower the rear of the headliner to gain access to the antenna
4. Loosen the antenna cable bolt and disconnect the cable from the antenna
5. Remove the antenna mast if equipped
6. Press the two tabs (underneath) and remove the antenna.
Hope this helps. It can't be removed from the outside, unless you use a big chisel and a hammer!!!!! LOL
Personally, I felt that BLACK antenna bases with a short mast look cheap on our cars. That's why I replaced.
Sorry I offended anyone with my suggestion.