twiget,
I think Rare81 is right about the plastic antenna strip/link being broken. It's a known problem with the C4's, with the tip off being just what you are experiencing - the antenna motor runs, but the antenna doesn't go down.
The replacement masts/with plastic strip can be ordered from Mid America
http://www.madirect.com/ Part# 601-244 @ $39.99 according to the catalog I'm looking at and it comes with complete instructions.
If your lucky, do what Rare81 suggests for installing the new one. If the plastic strip isn't broken off inside the motor, which is unlikely, it should work.
Here's some more info on replacing the antenna mast :
You first need to remove the antenna mast retaining nut on the rear fender that the mast feeds out of.
Then turn on the radio, works better with 2 people
.
As you antenna feeds out, catch it, and continue to let the plastic toothed cable feed out.
Then take the new mast with plastic toothed cable and hand feed the cable into the antenna hole. When you feel it contact the gear in the motor, rotate the cable until you feel it lock into the gear.
Then, while holding the cable and mast straight, turn the radio off. If the cable is inserted into the gear correctly it will feed in, taking most of the antenna with it.
Put the antenna mast retaining nut back on over the new mast and tighten it down.
Then turn the radio on and then off, and the mast should adjust and reseat itself all the way down.