I was about to say: Don't rule out the switch just yet. It has multiple contacts for various functions so even if other systems respond to it (such as your cruise control), the brake lamp circuit may be faulty. Makes sense since it's used much more often than the cruise control.
I replaced the brake switch in mine a week ago because of the opposite- the brake lights worked fine but the cruise worked intermittently. It would not activate at all, then activate but turn itself off then, after it "warmed up", would stay on... mostly. The cruise probably failed because mine isn't driven often at all so those contacts probably corroded over but the more often used brake light contacts stayed clean from normal use. I replaced the switch and just returned from an 1,800-mile round trip to Florida and the cruise worked perfectly every time.