Does it activate but not hold speed? If it activates, the servo is good, the brain is dead. If the servo does not activate, check to see if there is any voltage to it.

Usually CC's have their own speed sensors, they don't use the speedo.

Hell, on MB's of vintage, if you put in the wrong taillight bulb, it would mess up the brain, its hooked into the brake light circuit. Sounds like Urban Legend, but I heard it several times.

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MB-less