Originally Posted by WayneJ
Your bike either has a dying battery, a bad connection somewhere, or the charging system is not working correctly.
I've seen posts saying that the XT doesn't have enough charging capacity to use heated gear without doing something to reduce electrical usage elsewhere. The people making those posts probably had weak batteries, a bad connection, or faulty charging systems.
I used to use a Widder heated vest on the 1992 XT I used to own and the bike never had a problem keeping the voltage up, no light dimming at all when using the vest and it was completely stock. Even when idling, and I verified this with a multimeter, it had 13+ volts.
Perhaps your vest by itself didn't pull too much power, also I believe your year had a 35 watt bulb, rather than a 55 as the later years did. I know the earlier models had a less powerful charging system, but maybe it was enough with the lower wattage bulb.
If I ride with my full jacket liner and heated gloves on, they will eventually kill even a new battery. If I shut them both off a few miles before I stop, the battery will fully recharge itself, so I don't see any issues with the charging system or battery.
I believe the system is adequate for it's intended purposes, but not sufficient for additional heavy power uses.