Well, the only other thing that I have ever replaced back there was the two stabalizers under the third seat.....have you checked the tightness on the two large black rubber seats on the stabalizers....

Good Luck and If you are interested, I have the following that was custom made...

In my 1995 E320 Wagon I had a custom box made with a 10" subwoofer, spot for CD unit and large pioneer amp.... I only have the sub woofer left in the box as I do not want to crack it open....anyway it takes the place of the resevoir cover....