(I just had this thread on detailing -- no responses -- and a buddy suggested this section instead. Dohhh! Why didn't I think of that?!)
On my '81 300SD, my antenna was partially broken -- the motor still worked, raised and lowered in as far as it would go, limited by the part that was bent over/broke.
I bought a new mast assembly for $30. It has the same nylon tail, as the orignal with the teeth on one side to be grasped by the toothed gear wheel. I can't get it to work. I first held back the spring on the gear wheel with some needle nose vice grips and fed the nylon into the chamber. It was kinda tough, but I got it in there. Then, I plugged it in and it only raised a bit. I tried a different tack, pulling the telescoping mast all the way out, and feeding the end of the nylon into the gear. I had prepared the electric situation by plugging in the antenna motor, turning on the radio, then unplugging the antenna with the gear box in the extended position, thinking that I could then just plug it back in, with the nylon tip just started, and it would retract in. It didn't, except about 1/3 rd of total.
I then coiled the nylon strap up into a tight circle, and blue taped it for a couple of weeks, hoping that it was the difficulty in coiling the virgin strap (easy now) that was the problem. Still no luck.
Anybody done this before?