Sounds like maybe a short under the dash. The indicator normally gets power when the switch sends power to the high beam (the indicator light is on the same circuit). If the indicator is getting power when on low beam, its either a faulty switch (in which case you'd have high and low beams on together) or a dash short that is providing power to the indicator (and the high beam is drawing some power too - since its in the same circuit as the high beam - hence the dash lights get dim.)
Jim
