YOu said you have pulled it apart ? is anything touching the sides or the switch back exposed or touching ?
My issue was a little differant in that bike i borrowed last week, it flattend 2 boxes in as many heats both had been rebuilt i assume just the batteries, they would run for 5 mins then the light went out and was flat, i noticed that 4 pin plug that goes into them was loose and keep falling out and assume its soldered somehow and may be earthing to itself or similar and draining out, if your getting a kick from the switch it must be live and your the earth so there must be wirirng issues in that switch, can you change the switch out.