I put in a led tail light and it seemed to help. But it is common on sleds. You can check your stator to make sure. Do some searching and you will find good info about checking stator.
2000 on up had a lot more juice available. I'm afraid your 99 is going to do that regardless. You can try an L.E.D. tail light, or try running your grip heaters lower.
What type of led taillight are you guys using, i got one last year- the kind that has a bunch of little bulbs but it was only about half as bright as standard bulb, so i put the old one back in.