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there are MANY things that can cause this....
bad brake light switch
bad clutch or transmission position switch
failed ignition switch
failed engine computer
power distribution failure
if he has had the engine open twice and has been all over the engine compartment, it is very hard to guess what he might have done or what problem might have been caused.
this kind of concern is best diagnosed with a factory MINI/BMW scan tool which can read faults and live data from all systems. So, if this mechanic does not have such a scan tool, I would suggest moving the car elsewhere
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