I wanted to chime in with some additional tips as I went through my own window regular repair over the weekend. The local Ferrari/Maserati dealer quoted me $900 ($650 part + $250 labor) so I opted to give it my own shot.
The door panel is very easy to remove and requires about 20 minutes with a Phillips head screw driver and disconnect / removal of the door speakers.
Once the panel is removed you'll note that there is a very thin piece of Styrofoam/plastic insulating the door panel. You can peel this carefully without tearing but truthfully I don't think you need it. I saved mine for later and just taped it back into place - there is no need to attempt re-gluing it for any reason.
With all that aside inspect your window and the window regulator. If you're window regulator is blown out or off track then your window will not raise or lower at all. The toothed arm of the regulator on mine had some how popped up above actual motor gear saw the window was nothing to move the gear up and down. This created tension on the bearing that connects both of the regulator arms and blew out a small washer. At the dealer this would require replacement and I have no idea how to weld nor the tools.
The window however was actually still on track - which is separate from the regulator and you do not need to take the window off the track or you will have to readjust it later. Instead, remove the two bolts (the middle two of the four total on the bottom of the window) that connect the window regulator to the window itself. Next, push up on the bottom of the window until it is raised enough that you can grab it by hand and pull it all the way up. In a completely raised position place a C-clamp on the track to prevent the window from lowering. In this way it will stay out of your way and you will not have to adjust anything later. I noted at this point that the toothed arm had scratched some of the tint from my window - minor but certainly possible if you've driven around at all with the window fallen inside the door.
Loosen and remove the four bolts that mount the window regulator to the door (being careful not to let the assembly drop by holding it with one hand from the inside). Remove the two bolts connecting the rail from the door as well.
Removing the assembly from the door is a bit of a puzzle but move it around until you find the right angle - it it comes apart, do not worry, just remove each piece. Note that the motor has two cables attached and they're fairly short so they'll just reach the edge of the opening and you need to unclip them in order to completely remove the assembly.
Now, reconnect the two arms by first clipping the toothed arm back into the gear (about mid-way, no need to record a position or anything here), then sliding the bearing back through the hole in the other arm. Replace the spring by fitting the center into the notch in the bearing and sliding the hooked portion over the metal tab on the arm (use a screw driver to push it back if necessary).
Now, rather then replacing the blown washer by welding I chose a more mechanical approach that will not require much effort or tools:
Take two of the short screws (that mounted the assembly to the door frame) to your local hardware store and match the thread but purchase longer replacement screws - about 2+ inches (and obviously metric). You will need the following to fabricate a replacement:
* Two replacement screws (2+ inches)
* 6 matching nuts
* 2 lock washers
* A length of metal - any hardware store sells metal strips for about $8 (usually a few feet long and varying widths). I purchased a 4 ft. piece of aluminum (that was the shortest available) that was about 1 in. wide and a sturdy thickness.
I then placed the aluminum piece over the two screw holes nearest the assembly bearing and marked the two holes, cut to length (about 4.5"), and drilled holes just large enough for the screws to pass through.
I then fed the bolts through the metal strip (reverse of the original orientation to make re-installation easier) with a lock washer (on the side of the screw head), and a nut to lock the metal strip in place. I screwed another nut on and screwed the bolt back through the opposite side of the assembly. Once this was complete, I tightened the second nut to prevent any slipping. Once this is complete cut the screws if necessary so they do not stick out too far (but remember to allow enough room to pass through the door still and tighten with a washer and one additional nut.
* You'll have to cut a notch in one of the arms to allow it to rotate about 1/4" past the screw. Once you put the screws in place simply rotate the assembly on the bearing and you'll see where to cut this. It does not need to be exact and there is plenty of material so as not to worry about losing stability in the arm.
Here is the finished product:
With the assembly fixed replace back into the door panel - again this is a bit of a puzzle. I found that this was easiest with the toothed arm set about mid-way and sliding the rail (on the assembly wheel) into a position that allowed me to fit the assembly back into the door at an angle. If you need to adjust the position of the toothed arm simply reconnect the main power (the the blue connector ring) and use your window up/down switch.
Once the assembly is back in the door match the two large screws with the correct holes, replace the original washers, and tighten the nuts. Replace the remaining screws re-connect the power.
At this point turn the power on and press the window down switch until the assembly is returned to a completely lowered position. Holding the window by hand, remove the C-clamp and carefully lower the window back onto the assembly. Reconnect and tighten the two bolts to hold the window in place.
At this point do a full test. Here is how it should look (pardon the expletive, we were sending the video to someone who thought we were foolish to attempt to repair a Maserati in an effort to save $900):
After that, put your door panel back on and enjoy! All said and done the parts cost less than $10 (versus $900 for a dealer fix) and our tests indicates this was stronger and more reliable. And, no, there is zero scraping of metal or any other friction. I suspect this solution will outlive the car - and certainly another window assembly. Tools required were a Phillips head (for the door panel), a ratchet (8mm and 10mm I believe), a Dremel, and a metal saw (easiest way to cut the aluminum strip and screws).