You have a broken track holder on your window regulator. This is the most common failure among 300E owners as these age. I had replaced the regulators on both rear doors within a year of each other some time ago.
I believe the part is obtainable for a couple of dollars. Otherwise, depending on how much playing with the window you've already done, you may have buggered up the cable mechanism enough to warrant a new assembly ($140+).
Either way, you need to remove the door panel to get to the offending part. It's about an hour (or more if it's your first time) to do the R&R...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car