I suppose the "solution" varies from model to model but here is what I did to fix my driver side SQUEAKY window. First let me preface this by saying my passenger door window did the same thing a couple of years ago. Took it to the Mercedes dealer, and one new window regulator later, the problem was solved. This time however, after reading where some guys were able to solve the problem on their own, I decided to give it a whirl. Now, when I say my window squeaked, it was like nails on a chalkboard! Absolutely horrible. Did I say horrible?!! It was the worst just as it would start to go down and then lighten up about half way down. It wasn't nearly as bad on the way up however - no idea why that could be. Anyway, I took the whole door panel off. If you need instructions on how to do that, there is an excellent YouTube video showing every step. Once the panel was off, I also took out the main door speaker, which allowed for a better view of the right side rail of the regulator. I greased everything I could. At first I thought the problem was the left and right side vertical rails of the regulator. I greased both sides of where the plastic guides (blue in my case) slid up and down the edge of the rail - waste of time. I even went as far as greasing the inside of the rubber that the window slides up and down in, located closest to the far right side of inside the door - Nothing! Ugh. I didn't know what else to do. I then thought that perhaps its somehow the steel cable inside the sleeve but there really was no way of accessing that but then I thought that cable must be on some pulleys. If you reach your fingers around, you can feel the pulleys that the cable slides around. Using a can of Slick 50 spray lube, I did the best I could to saturate those pulleys (the pulley is attached to the backside of the metal rail so its guesswork trying to hit your target, soooo I aimed the can as best I could and sprayed away. I was actually able to see the far right pulley by looking through the opening in the door handle area, so that was able to get a direct shot. This was my hail mary pass, otherwise game over. AND IT WORKED! It immediately had a big improvement and after going up and down about 5 times, the squeal was completely gone. Time for a beer! Thank you, thank you very much.