If just the external mast is broken, and the part that engages the motor is
still intact, you won't need access to the motor.
Here is how to do it:

1) Unsnap the plastic cap (careful not to break the tabs -- though one or
two may break anyway because of age)
2) You will see a nut. Remove it.
3) Turn ignition and radio on to extend the antenna.
4) Firmly pull out the old mast and cable from the motor.
5) Check that there is a taper at the end of the plastic cable. If there
isn't, the cable is broken, and you'll need to take the motor out and remove
the broken piece.
6) Position the new cable with the teeth facing the front of the vehicle.
Manually extend the antenna and insert the cable until resistance is felt.
Turn slightly left and right to fully engage in the motor gear.
7) Have someone turn the radio off and apply light pressure to the antenna
to help the motor engage and retract it. Guide it in, as the motor pulls.
8) Install the nut and the cap above.
9) Cycle radio power on and off a few times to verify the antenna is
operating smoothly.

If you are unlucky enough to need access to the motor:
after removing the nut and the mast:
1) Remove the single screw at the bottom of the motor under the fender
2) Remove RH cowl trim panel (inside vehicle)
3) Disconnect electrical connector
4) Press the tabs and drop the glove compartment door
5) Disconnect antenna cable and push cable into engine compartment
6) Move motor forward and remove
7) Remove cover and throw away the broken off mast cable piece

Install in reverse order