It's probably your clutch switch. I had a similar problem with my car, occasionally it would start...sometimes I'd turn the key and nothing would happen...no click, no nothing. If it wasn't starting, I'd just let the clutch out a fraction and turn the key...would start right up. My problem went away all by itself over time.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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