Missing tooth on the white nylon gear that pulls the nylon strip down?
If it gets to the sticking point, and you push it down further while the
motor is still running, does it grab and continue down?

The other possbility is that one of the antenna tube sections, which are
supposed to be perfectly round, has gotten slightly crushed. A slightly
oval section won't pull into the perfectly circular section below it.

Bill
------------------

You can remove the splash shield from the right wheel well and there
are three 10mm bolts that hold it in.  It's pretty straight forward
once you get it out.  I have attached a post I downloaded about
6months ago when I did mine.  It took about an hour to complete and I
had to remove the motor to get the broken drive cable out.  The new
antenna cost about $33.45 plus shipping from ford-parts.com.

This is from the repair CD:

Automatic Electric Antenna

Removal

1. Using an appropriate tool, unsnap the cap from the antenna base and
remove the radio antenna base cap (18A927).

2. Remove radio antenna nut (18865).

3. Turn the ignition switch (11572) to the ACC position and turn the
radio chassis (18806) on to extend the electric antenna.

4. Firmly pull the electric antenna out of the electric antenna motor.

Installation

1. Check the plastic cable end for a slight taper. If the cable
appears to be broken, a section of the plastic cable could be inside
the electric antenna motor. To remove the cable, proceed to Removal
and Installation, Antenna Motor, Electric. If the cable is not broken,
continue with Step 2.

2. Obtain a new electric antenna. Position the plastic cable so the
teeth are facing the front of the vehicle.

3. Manually extend the electric antenna, then insert the cable into
the electric antenna motor until resistance is felt. Turn the cable
slightly left and right to fully engage the cable into the motor gear.

4. Have an assistant turn the ignition switch to the ON position, then
turn the radio chassis ON and OFF. Apply light downward pressure to
the electric antenna as the electric antenna motor starts retracting.
The electric antenna motor will wind the cable into the electric
antenna motor drive unit and retract the electric antenna sections.

5. Install radio antenna nut. Tighten to 9.5-10.5 N-m (84-93 lb-in).

6. Install the radio antenna base cap.

7. Cycle the electric antenna motor several times to ensure the
electric antenna is extending and retracting properly.
   

    I'm including this from the CD incase you need to pull the
whole antenna:

Antenna Motor, Electric

Removal

1. Lower electric antenna (18850).

2. Remove retaining screws of RH front wheel splash shield for access
to antenna lower retaining screw.

3. Remove lower retaining screw.

4. Using appropriate fender protection, unsnap radio antenna base cap
(18A927) from antenna base and remove.

5. Remove radio antenna nut (18865) from top of motor tube and remove
electric antenna through fender/splash shield access.

6. Disconnect radio antenna lead in cable (18812) from electric
antenna.

7. Disconnect motor wire connector from electric antenna.

8. If a section of plastic antenna cable is to be removed from antenna
motor:

l Remove the four attaching screws, drive mechanism cover and drive
mechanism.

l Locate and remove the broken piece of cable.

l Install the drive mechanism, drive mechanism cover, and attaching
screws.

Installation

1. To install, reverse removal procedure.

2. Tighten top radio antenna nut 9.5-10.5 N-m (84-93 lb-in).

3. Tighten lower electric antenna retaining screw to 5-6 N-m (44-53
lb-in).