I had an identical problem with my '80 300SD. I purchased a used window regulator from a wrecked '74 450SEL. I did the job myself and I do remember it being somewhat of a pain. The motor does need to be detached from the regulator. It is done more easily by inserting your ratchet into the three small holes the other writer indicated.
I was able to successfully replace mine. However, I do remember that the replaced one never seemed to go up as smoothly, or maybe as far as the other. Good luck.