Gus, although I've never worked on an SL500, based on other MB automobiles, I suspect you need to remove the trunk compartment liner panels on each side. Typically the side-to-side panel locks the side panels in place so you need to remove it first. Then release the side panels from the upper clips and twist/lift the panels out of the trunk. Once you do one side the other will be easier.

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Fred Hoelzle