Locke rejected what is essentially Kant's categorical imperative: “that one should do only as he would be done unto” (Essay 1, Kindle Locations 650-652). The rejection was based on the fact that the moral tenet does not demand the consent of reason, after all it is not unreasonable to question why we should obey the imperative.
From within Kant's theory of understanding as providing all objects and all relations between objects via apperception, and his theory of reason as supervening upon the stream of consciousness appercieved Locke's claim is justified. Only when Kant steps into the transcendental and argues for the reasonableness of morality based upon its occurrence in the noumenal realm does morality regain its demand upon the consent of reason; it is the noumenal objects that are involved in the initial meeting and they determine the will in the noumenal realm, which noumenal realm being nothing but logical possibilities syllogistically deduced from the absolute Idea or transcendental major premise means the initial determination of the will is determined by reason.
But this argument only goes to justify why there is morality in the world irrespective of the fact that it appears unreasonable for it to be there. It does not justify why Kant would identify the noumenal process determining the will (which determination will always be moral) with the categorical imperative as our phenomenal "guess" at what that noumenal process is. I know of no argument Kant makes to this identification. Does someone know of a passage?