2012 Hyundai Sonata Headlight Switch

These symptoms are typical of a bad ground wire for the rear lights. You'll find with the head lights off, the brake lights really aren't working properly because they're not as bright as they should be. Current is indeed going through the brake light bulbs, but to get to ground it has to also go through the tail light filaments. That reduces their brightness but that's often hard to notice. Turning on the head lights / running lights removes that ground path for the brake lights so they stop working. The loss of the ground puts voltage from the tail lights onto the brake light circuit, and that voltage is what the cruise control system sees and thinks you're pressing the brake pedal. That's its cue to kick out and stop working.