Not sure about your 92 300E, but it may be similar to the shelf area on my 81 300D. Just did all this few days ago; reinstalled yesterday. To remove the shelf on my 300D, one must remove speakers by removing screws in the speaker covers, remove first aid box by removing two screws in front edge under lid, remove upholstered pillars on each side, then lift the carpeted shelf slightly and pull it forward and out. The metal part of the shelf underneath the carpeted shelf cover, however, is formed with and/or welded to the rest of the surrounding sheetmetal - horizontal shelf bends downward and forms sheetmetal between seat backrest and fuel tank; at the floor, this piece of metal becomes the floor underneath the seat, etc.
One can see underside of the metal shelf in the trunk, but I didn't see any reasonable way to remove it without torch, saw, etc., but I was not trying to remove it and did not consider it - there may be some "magic" something there that I did not see - doesn't seem very feasible to me. 92 300E also may be quite different.
91 350 SDL. 81 300D