Nice Info!Cleaning the gunk out is only a temporary fix for this problem. If you want to fix the motor properly you MUST have the 2 carbon brushes replaced. That's what they do when they recondition it for resale.
The 2 carbon brushes are quite soft in this motor. That's what all the gunk is. Carbon dust build up. When the brushes get worn down they become too short which reduced the ability of the springs to keep them pressed up against the contact point, hence the motor stops working. Thump on the inner door skin jolts them back against the contacts to work again but not for long.
Replacing the bushes is quite easy to do. Dig out the silicon on the side of the motor to expose the 2 electrical wires. Then heat up the 2 electrical pins (with a soldering iron) that the wires attach to enough to melt the solder to detach the wires. This will allow you to then pull apart the unit to get to the springs & brushes.
The problem has nothing to do with weak springs. Its just that the carbon brushes wear out & become too short & no longer make contact with enough pressure to conduct the electrical circuit.
Take the old carbon brushes to an electrical supplier to buy suitable replacements. I could only find ones similar in size but they were quite easy to file down to the correct size by rubbing them on emery paper. (the brushes are a bit like led in a pencil) Some places will make them for you.
Detach the old brushes by cutting the small braided copper connectors (don't pull them off the metal base as it is harder to solder on the replacements) better to cut the copper braid mid way & solder on the new ones. (much easier to solder copper to copper).
Now your window motors are as good as new.
All up cost me a few dollars & a bit of my time. Much cheaper than the $800 - $900 NISSAN in Australia charge!!!
NB Once the window motor has been disconnected (from the computer) you will need to reconfigure the window mechanism (reprogram the top & bottom positions). (The sequence for this has been well covered before).
Hope this adds the final piece of the puzzel to all re this frustrating problem.