1st check you third brake light bulbs. You may have a couple of bulbs with bouncing broken filaments making intermittent contact. If the 3rd brake light is not working properly-you cruise will not also. They are connected through your brake light switch under the top of your brake peddle.
If all the lights are fine, next is to check the brake light switch. The switch is made of plastic and works with a little plastic lever connected to the upper brake peddle.
I know this because my brake light switch started to go bad. Cruise stopped working, as well as the shifter interlock famous "click" stopped when stepping on the brakes when first starting the car. Switched was changed-problem never came back. Cruise works again & "click" is there again like it should be.
Your SS switch is a little different than my LS, here is yours:
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