Originally Posted by BanjoBoy
Have him check the brake light switches.
Make sure you can hear the "click" of the switch and that brake light goes on when front and rear brake levers are actuated.
You wrote that the bulb is lit, but I am not sure if you meant warning light or the rear brake light.
Did the bike stall in the mud? That is, did the warning light come on while the bike was still running, or was it after restart?