3M makes an acrylic mounting tape that's used to mount body side moldings and badges to car bodies. Once stuck and allowed to cure, it's designed to be permanent. Available at most auto parts stores and auto body supply shops. Buy some, cut the width that you need with a razor knife, stick it to the light housing (cleaned, of course). Then, after cleaning the glass well, stick the whole assembly on. It has to cure for 24 hours. After that, you can remove it only with a razor blade. Works wonderfully.

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