The black plastic, or blackened glass in some cases, is just the exterior where the back pillar is. For some reason, it is in plastic or glass instead of steel. Maybe for glass replacement ease. Anyway, you are right, it makes a convenient mounting point. I wonder if the plastic mouldings could be removed and their mounting points used for a spoiler. I wonder if substntial amounts of double sided trim tape could be part of a mount to glass.
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See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.
Speaking purely from a speculative point of view; that appears to be a perfect location for a Coanda nozzle:
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.Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster
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.Three conspiracy theorists walk into a bar --You can't say that is a coincidence.