Just fixed both front windows on my 126 300 SD. Look at the bottom track of the glass. At the end of the lift bars you will see the plastic guide. If it is broken you will have to follow the previous instructions. However it could be the motor. They are known to sieze up as in my case. Remove the entire lift mechanism. It is held to the inner skin by bolts which are all visible. You can remove the entire mechanism from the door. When you get it out remove the two hex bolts that hold the motor to the gear mechanism and separate them. There are two rubber plugs on the motor case. Remove and saturate with transmission fluid and manually turn the spiral gear. Hook up the motor to a battery to see if it runs. Reversing the leads reverses the motor. 95% of the time this will free up the motor. Pack the spiral gear with lithium grease and reinstall.
Good luck
John
The first window took an hour. The second one took 20 minutes.

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John Olszewski

1982 380 SEL (Euro)
1984 300SD
1986 300E (Euro)
1990 300SL