I finally fixed this! What happens is that the ribbed nylon tail that goes thru a gear and wraps around inside the case breaks and will no longer pull down the mast. You need to get a new mast and replace it. I had to take the entire antenna assembly out of the car, then take the five or six screws (don't forget the one in the center of the round part) out of the case and separate the two sides of the case. Then you lift off the round white part and pull out the old nylon pieces and generally clean it up. At that point you should be able to pull the old mast assembly out the top.
According to the book, you should be able to reassemble the unit and reinstall in the car before you put in the new mast (LEAVE THE TOP OUTSIDE SCREW PIECE OFF UNTIL MAST REPLACED). First you turn the radio on, which would extend the mast if it was there. When the motor stops, you feed the nylon end of the new mast down the hole as far as it will go, with the ribbed side to the front of the car, If the end is engaging the little screw gear, when you have someone else turn the radio off it's supposed to suck the new antenna right in. It may take several tries to get it right. I have successfully done this on other cars, but not on this one.
Plan B is to reinstall the mast outside the car before you put the two black case pieces back together. You put the round white piece back on and PUT THE SCREW BACK IN THE CENTER. Then you put power to the appropriate pins in the connector that power the motor - make sure your polarity is correct; you can find the pin info in the FSM on the power antennal page. (I use one of those jump batteries and a cig socket adaptor with alligator clips on the end.) The motor MIGHT run then if the unit thinks that the antena is "up." Next, you put power on the "detector" pin that receives the on/off signal from the radio. This is telling the circuit that you want the antenna "up" and it should turn the motor for awhile. Once it stops, you put the end of the new mast in thru the tube and down to where you can see it go in the opening where the little drive screw sits. Now, when you disconnect the power from the detector pin, the unit should start the motor and start pulling the white nylon into the area enclosed by the round plastic thing. Once it starts pulling, feed the mast into the hole and let it run until the motor circuit times it out. You can then disconnect all the power.
Once this is done successfully, you then CAREFULLY remove the center screw and put the black case top back on. You need to keep some pressure on the round white piece, because the coiled up nylon inside wants to spring it off. (I used a small phillips head screwdriver thru the appropriate hole in the black top case to hold pressure on the round white piece while I lowered the top back into place.) Put your screws back in the case and reinstall the assembly in the car.
Good luck - let us know how it goes. Sorry I don't have pics!