I have been asked to fix this common problem on lots of 99 on DT125R,s not the RE,s and X,s, its the loss of power to everything that should work when the ignition is switched on and the engine is not running.
Its an easy fix just split into your loom under the seat, the solderd joint from the Red battery power feed that splits and goes to the regulator and on upto the ignition switch suffer from dry solder joints. Just pull it apart and rejoin it.