Well, the speed of light is 186,282 m/s, so I guess if you set your cruise to 670,616,629 mph, then you might notice your headlight output not being as effective.
In seriousness, here's an excellent write-up from a gentleman who had exactly this problem and discovered that anything other than a ground on the brake line would cause his cruise control to disable. I don't know how the MC cruise is wired in(I don't have one), but hopefully this link will point you in a possible direction.
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