Mr basil. I do not even remember if your car has a rear sway bar. The bushings of it at points of contact might be worth looking at. The bushings or whatever can wear so bad you have steel on steel even. Somehow though I think that is not your problem. Yet if you get the squeek by pushing on either rear corner the sway bar is common to both sides. Do you have the fifth coil spring on your particular rear axel? I have not seen one of those for ages and cannot remember when they stopped using them.
Getting a young fellow to lay on the ground and listen when you exercise each corner can help identify the source of the squeek sometimes as well.
I am not experienced in these matters with mercedes yet your type of problem applies to all brands of cars. You have not introduced any new things into the trunk reciently have you?