[Mike Burnham]

Thee can be tricky, but I will try to point you ina direction. First you need to check for a short or grounding problem in the elctric lines to the light.
Next if this proves wrong, you need to check the neutral switch in the gear box. If this is to much work, then you need to see a shop manual to see the wire harness diagram. Chances are you may have to get a mechanic to look at it, and this may cost you some money. Sorry if this does not help much but with out the bike infront of me, this is the best I can do.