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You're over complicating it. Hook it up to the brake light switch. Actually hook it up to the cruise control terminal on the switch. Run a ground. Done.
Odds you will be braking from a 4-5 shift are slim and the 6-5 shift is straight in line.
I've done installs with auxiliary switch and the brake pedal. Both are viable options with the brake light one being the most mindless.
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