I had this problem some time ago and it may or may not be an easy fix.

I think that the lower part of the hollow threaded bolt (retainer) that secures the antenna mast, is broken off, and is still in the top of the antenna housing.

If it is, the mast can not be removed as the broken piece will prevent it from coming out. There is a rim or boss on the bottom of the last tube which keeps it in against the retainer.

You can not remove the broken retainer because the mast is in the way so you will have to cut the mast and pull out the plastic chord whitch pushes/pulls the metal tubes up or down.

Now you can use a tool called an "easy out" which has a reverse threaded screw tip, push the tube in to just bellow the opening, insert the correct size easy out into the hole, then slowly and carefully turn counter-clickwise and it will unscrew the broken retainer and hopefully also the last peice of the metal antenna tube.

Note: If you don't get the lower section of tubing out you will have to remove the antenna, disassemble it and push the tube out from the inside.

This all may sound more difficult than it actually is.

Good luck.

Acky