I had the same problem too. The innards of the switch just melted and locked the
switch in the 'ON' position.

That is why I carry an extra headlight switch in my Van now.
I think that this is just a result of how they designed the switch. I'm told it
carries the full current that goes to the headlights.

If so, the solution would be to rewire your headlights using a direct power wire
from the battery and a relay to the headlight switch. (I believe)

Todd
'88 Westy