One day i decided to lay across the back seat of my Toyota to use my computer, i wound the window down and... mash... didn't go down smoothly, didn't go up. Further attempts was destroying the plastic inside...

I am not sure of other cars out there, but inspecting and being creative might save yourself and others some $$$ (unless 3 days for a perfect solution is too long for you!).

After my first inspection and attempt at fixing, i thought it was a lousy design (despite lasting 20+ years, although did not have constant use) and thought that if i wanted to fix it right i would have to design a new one (would have been something that had larger dimensions!). But i started getting some inspiration before sleep, and decided to persist.

I didn't get it right the first time - If you have to do it different, be sure to test a few times before reassembling the door. I kept notes because i have four doors.

Parts list:

appropriate tools to dismantle the door and remove the regulator (cloth to remove winding handle, phillips screwdriver, 10mm...), and to keep the window up.
You will also need a grasping tool (long nose pliers)
Hammer, gardener's gloves (optional), permanent marker
Cheapo bag tie that has a fine wire within. Sold with food bags.
very small drill bit as well as drill or dremel-like tool (preferably).

Step 1: Pull Window Up

You need to do this to remove the window regulator and to repair (and to keep the weather out). You will have to fight it to get the window up to the point that you can pull the regulator out.

Part B: Please do the next step before this part: After pulling on the Up cable to pull it up properly, you will need something to keep it up otherwise it creeps down.

Some options:

Wedge the window up (the window was not flat underneath, this allowed me to slip a tool's hand grip into the top of the door, and wedged it in using the regulator handle against the glass).
Sticky tape (from the outside while the window held up by other means, ie, vice grips on the metal cable): Stretch a long length of tape from the bottom of the window to the top and go over the top of the window frame.
Otherwise try to get a prop in there to hold it up, maybe your car will give you other options. The vice-grips on the wire against its sleeve will be fine while you have to drive the car (but tie them to the door) and is the only real burgle-proof way, but this wont work while fixing.

Then pull both cables hard to make sure there is no hidden slack (i forgot for my final repair!) Leaving slack will not be detrimental, just means the slack will move before the window moves when you turn the handle.

Step 2: Fix the Damage

Remove the regulator and disassemble: hit the bolts out, pull out the tangled mess of cable, please note the orientation of the spool with the marker before lifting it up from the disc (If the disc is highly polished, it may be better to scratch, then mark the scratch). untangle the mess of cable.

The wire will be all bent, straighten. The wire that is still mounted on the spool (which didn't fall off) has a last bend, this need not be straightened.

Inspect the spool, it has guides molded, these are likely stripped, remove the frayed/cut material.

Step 3: Remove the Disc and Its Spring From the Shell

The most dangerous part is the spring, despite holding no tension it still holds alot of energy while it sits confined.

It is also capable of slicing (remember that while inserting, you may need gardeng or leather gloves).

You will have to pull the disc out of the shell. The safest way to remove the disc is to rotate it in the "wrong" (UP) direction, the end of the spring should slip out of the slot in the center tube (not sure how good it will do this with the factory's bend) I suggest to do it with the aluminium spline-thing down the middle to prevent the end going into the middle. If done without and it slips into the middle, push it out and try again, or use the pliers to pull the disc up by the end. Just pulling the disc up may release the spring, but if it doesn't, try not to force it out because that will bend the end and you will have to reform it, and you will not be able to do as good job (my fix made the last bend a bit longer and it rubbed on the part that goes down the middle).

Once the disc is out, use the pliers to pull the middle out and let it shoot out - hold it away and pull it away from you.

Step 4: Drill Hole in the Disc

One hole, i chose like 3-4mm from the where the barrel head (end of wire) is pushed in, in the direction the cable will go towards the spool. My hole is 1.5mm but it could be smaller.

Then cut a trench in the disc from the hole to where the barrel gets pushed in, this is for the wire to sit in to prevent contact with the spring (Second image shows the thicker wire weighing down the thin wire, showing that the thin wire will recess into the plastic).

I used the drill bit in the dremel-like tool, knowing the drill bit "climbs" as you do this, also turn the disc 180° and cut a little more so the other wall is steeper.

Step 5: Push Barrel In, Tie

Pull the thin wire from its plastic, bend in half. These bag ties are good, they have worked well to hold my glasses frames together.

Ensure all the bits (the spring) that need to be on the UP cable are on it.

Push the barrel (end of UP cable) into where it should go, then push the two ends of the wire through hole but on either side of cable, pull through to spring side, fold them over your track and then thread both ends to the top side right beside the barrel (have the ends come through either side of the cable), and then twist. Make sure the wire runs inside track and pull the cable up this will pull out the slack in the wire and you can tighten some more (you can use the long nose pliers to get the turns closer, but don't overdo it by making the turns too tight together or you will need another wire), cut appropriately, push the end beside the cable. Ensure the wire is properly in the track or you will be back for a re-repair some time later.

Step 6: Push the Spring in - Carefully

Push and hold the end in, guide the rest around once or twice, then shift to the pictured position. You could have the disc in your palm and use your fingers to hold it in (but you may require longer fingers).

Step 7: Trial and No Error

To do it the most safe way, don't bother tensioning the spring, i needed 5 rotations but it was both dangerous and hard to get to (i decided i needed cable ties and holes drilled in the shell, but i lost my grip and never got to use them), plus when the spring pulls itself loose, it also attempts to pull off the cable i tied down. And I couldn't tension it with the window lever due to the direction of the teeth on the disc. That is why i said to keep the window UP - much easier!

You can do it without pretensioning the spring but i have a feeling one rotation might have been still acceptable - so optionally rotate (in this case on the left-hand side, anticlockwise) once (going the wrong way will pull the spring out), then place the spool on (lining up the marks made with the pen), lay all of the Up cable onto the spool (don't wind the spool) and then lay it into its entry slot with a pull on the spool - this will always mean there is at least some tension on the spring.

The Down cable is then laid into its entry slot, apply pressure to the spool so the Up wire is tight and then start winding at the first unused cable guide, continue around until you reach the bend in the cable which should go (hopefully) in and push the barrel into the top of the spool. If you had no idea that the spool had so many orientations (and didn't mark it) you will have to keep trying until you get it tight - Note how far in whichever direction, disassemble and try again.

If you know it should go in but you cannot manage it in this orientation, try doing it left handed.
If you don't want to bother making it tight (or you never actually pulled all the cable through before tightening), the window will still go up and stay up, but if the user wants to wind up or down, the slack is moved first before the window moves.

If the window begins to whistle after you wind it up, it may be because the material the glass pushes against or rubs past is old or worn off (i will have to think of a way to fix). Perhaps the thicker spring in the front metal shell has slipped back. Try increasing its friction (wipe some of its grease out?). These other things will not improve it so much: Having that extra turn on the spring, or orienting the regulator handle so that coming down also means lifting the handle up (when reinstalling the handle, it should be up and away from you so you dont bump it).