It could be corroded or furred up connectors somewhere in the loom?

Although the earlier XTR/X were well known for the switch pin/plunger being too long, so the light either didn't illuminate at all, or did so very late on in the lever's travel. Either take the lever off so the switch plunger can fully extend, or remove the switch (keep it connected) and test it off the the brake perch. If it works like this, then you'll need to file down the end of the plunger so contact is made earlier, if it still doesn't work then check the wiring/connectors.

Kev create a post about this problem a few years back...

http://www.xt660.com/showpost.php?p=125785&postcount=6

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