Hey Derek, thanks for the response. When I was looking in there I did see some fasteners that looked to be passing through to the inner fender. I did think "How odd, why isn't it welded together rather than screwed?" It didn't occur to me that there may be a removable panel. The inside of the cavity is somewhat dirty and dusty but it doesn't look like there has been any water intrusion.

My plan was to trim up the speaker holes so that they are a more regular shape like a nice clean circle or rectangle with rounded corners or whatever. Then I was going to cut out a sheet metal cover that can be screwed on over the hole with a homemade black foam gasket sandwiched between. The thought was that being able to access the inside of the cavity could be beneficial in the future. Why? Heck, I don't know. Now I'm thinking I don't have to be all that particular about how well the cover plate is sealed against the body panel.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)