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I used heavy duty construction adhesive to fasten a 1 inch thick piece of wood to the headliner that is painted with Rustoleum Topside paint. The color is Oyster White, which is almost a perfect color match to the interior. That wood provides a secure place for screws to hold the radio bracket. There really isnt enough thickness in the fiberglass at that location, and I didnt want to thru bolt it.
All in all, I think this makes a nice clean installation.