There is a locking ring which needs to be carefully removed from the top of the antenna mast where it goes into the fender. The locking ring has two deep slots in it and is turned counter clock wise to loosen. Turn the ignition key on and the radio, with that ring removed you should be able to pull the cable and mast assembly out. Compare what you have withdrawn with the extended length of the old mast and cable, if they are the same length you can reinsert the new cable if not you may need to remove the inner black plastic fender liner to access the motor, disassemble it and remove the remaining broken cable in the motor.
If all the cable is withdrawn get a friend to turn off the ignition as you enter the new cable./mast into the electric antenna motor. If the cable doesn't fully engage with draw the cable and repeat, the process.