When I was working on the Ford racing program, I got into this discussion with someone who had been inside the Detroit scene for some time.
The way he explained it. . .and again. . . taking with a grain of salt?. . . is that the designers flitted about manufacturers so much, they took their ideas wherever they went.
Sort of like a bee implanting the same DNA pollen from plant to plant. In this case the same designer DNA from manufacturer to manufacturer.
I do find it difficult to believe that the similarities in design are purely coincidental.
That there has also been much consolidation among marques might also be a contributing factor.
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