I'm not familiar with the newer mustangs, but look on your brake pedal arm, about half way up should be an adjustable brake light switch, make sure that it's adjusted and working properly. You never know what else that switch has control over.
306 with all bolt ons, letter cam, ported E7's, T5 swap, full exhaust, 3:55's, 2" drop, and the worst paint you'll ever see or feel.
Dyno'd 300/316, 2745lbs-track scale
Best ET………12.4@108, 1.66 60ft on 8" ET Drags.