It's the guide or plastic cable pulley as I like to call it. These can warp with age and the cable can derail. That's what happened to my rear right door on the '96 car. I didn't feel like replacing the whole regulator so I just put everything back, carefully closed the window and I don't use it anymore because if I do the cable will probably derail again. MB really cheaped out with these plastic pulleys. I don't think using metal pulleys would have made the company go bankrupt.
BTW, what you have is an E300 Turbodiesel, also known as E300DT, not TD as that designation is for wagons, and I'm assuming yours is a sedan.