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Not sure, but I thought by law it had to have back up lights starting in 66 model year. Not positive on it though. I know the 68's had to have the side markers by federal mandate.
If by early 66 you mean a model year 65...then yes
otherwise, no.
Now I'm confused - well not really. All 1966 Mustangs, assembly starting in August of 1965, were equipped with back-up lights in accordance with federal requirements. It was an option in 1964 and 1965.
Jim