Ours might be a little different, but the speaker grills on our boat just pop off. You have to get something like an automotive trim remover tool to get behind the edge of the grill, then just slowly pry it off. When I replaced our speakers years ago, they were screwed into the fiberglass, but the covers just snapped onto the speakers.
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1999 180 Horizon OB Fisherman Pkg
San Antonio, TX
Choice of tow vehicle:
'93 Explorer 4x4 (sold)
'05 Grand Cherokee 4x4 w/ 5.7 Hemi (member of the 240k+ mile club!)