I am no expert but initially I thought that maybe your handbrake cable was moving, causing the 'on' light switch to trip.

But as zooming in on these three photos show, the switch must be located under or in front of the Hand Lever?

Maybe there is a short caused by Vibration or forward movement of the switch linkage? All guesswork, I'm afraid,
Frank









http://franks4x4-solograndetour.blogspot.co.uk/
Combat Stress-http://www.justgiving.com/Roadster100