I'll just throw in my 2cents. This happened w/ my 300E. A friend and I took the door apart and lubricated the window regulator (they were somewhat frozen up by the metal wire). This worked well for one. On the other, the motor failed with the window in the down position. Long story short, I ended up replacing the window regulator and motor (you have to replace both at the same time), but had to drive around w/ the window held up by a piece of wood.

My advice is to leave the back windows alone. The window regulators are a terrible design and them not working doesn't affect the driver at all.

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87 300E
202k on the clock