The antenna mast operates on a plastic cable with teeth on it. To remove the
old cable remove the nut on top
and turn the radio on and pull out the old cable. Remember the direction of
the teeth.
Measure the lenght of the old and new cables to  make sure you have all of
the old cable out,
if not you can access any broken off pieces by removing the cover on the
side of the antenna motor ...

To replace the new one stick the cable in the hole, turn radio off and the
motor will pull the cable and antenna
in and down and replace the nut.

cheers,
Igor