As long as the gears are still good in the motor I think I would replace the mast with a new one. For the amount of work is involved to remove the assembly I don't think it is worth converting to a fixed mast. I just replaced one in a wagon this week at work and as far as I can remember there are the screws on the rear sill plate that have to be removed, then a screw in the cargo net hook

at the top rear of the panel, and then I think there were 3 or 4 clips that secure the panel to the car. I did not have to completely remove the panel to get access. It should be flexible enough to get in there, just be careful not to break it since it's 13 yrs old. The assembly is held in by 2 10mm nuts, 1 hose at the bottom and the electrical connectors for the radio. This might seem like lots of work but I think it took about an hour to remove the power antenna assy, disassemble it to clean out the broken parts and reinstall the new one. One thing you may have to watch out for - the one I replaced had two different lengths available - so make sure you have the right one before you install it (I didn't). The longer one is about 1/4" longer than the other and it doesn't retract all the way. I hope this helps you out.
Shawn