Use these pics in conjunction with Bay 13's helpful instructions to replace the mast.
This is not a blow-by-blow series of instructions. That would be reinventing the wheel.
It is just a few helpful photos when you complete the process of replacing your mast, and may be more helpful when the belt is broken and remainders are left in the antenna motor body.

figure 1: the culprit
 

An initial note: you can probably do most of this in car without doing what I did below. But I wanted to see how it all went together and wanted to clean it properly, and it is quite easy to take it apart as long as you pay heed to where it all goes so that you can reassemble it properly.

There is a window-port from where you could do some cleaning and removal of old pieces of drive belt... but you miss the great adventure.

So assuming you have time to kill and like investigating your car, the info below might be useful.

Disconnect the whole power antenna assembly from the body. This requires an electrical unplugging and a removal of the small run-off pipe from the body of the car; but the pipe can stay on the antenna assembly.

  The space you must attack.
The drain pipe allowed to remain on the antenna motor body.

All of the bits and pieces.
 

Note carefully how it all goes together before you pull it apart. 
All of the surfaces should be cleaned up, as this is what causes the failure in the first place. 

I took mine part totally to clean it right up and then replaced all the lube with dry graphite powdered lubricant.