The only way is the hard way. Check the circuit for the headlamp bulb has any live connection . Unfortunatley the dashboard wiring on most British MV's was designed by a blind plumber. Check permenat powered leads, which lights should have, for alead that has frayed or come close to another lead, and clean all the contacts grime can cause power to leack,
The problem is proably the six way light switch, that has to be the worst single component ever designed for British MV's.