Thanks for attempting to help. I was finally able to open the speakers. It turns out that there is a deeply recessed pair of nuts that hold the knee bolster on on the driver's side. Once you release these, the bolster drops away and the hidden screws to which you refer are exposed. Removing them allows one to pull the grilles down and off. There are two clips behind the grille that hold the top od the grille in place; they are a real bugger to replace, which is why yours may have had glue used.

In any case, it turn out that my speakers were OK, so I didn't have to go on a quest to locate new ones.

Thanks, again,
Walt

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'87 560 SL