I would start with the basics, as was said the cruise control senses voltage in the brake light circuit to determine that you are stepping on the brake and so disengages the cruise control.
Start by observing the operation of the rear bulbs...hwat happens when you put on the turn signal? What happens when you put on the turn signal while your foot is on the brake? How the lights respond to instructions will give you an indication of whether there is a ground issue or possible a bulb issue.
Taking the steering wheel off and checking the stalks should be the last resort for this repair.