If your car has the original rear window lift mechanisms there is a good chance that they need to be replaced due to a broken nylon link in the lift cables. I had to replace the lift mechanisms in both rear doors on my wifes '93 300SD. The replacement parts are about $70.00 each and are a vast improvement with the nylon part being replaced with aluminum. The windows would creep down gradually hitting bumps. I would just press them up and they would stay in place due to friction in the tracks. When I finally found out how to get the door panels off without breaking anything I installed the new parts. Door panel removal has been covered on this forum in the past. Use the search function.