You're problem is almost deffinatly a faulty ground. With your lights on dim the voltage is "looking" for a ground, it has found one through the filament of your indicator bulb. The risistance of the filament in the headlight beam drops the voltage to the indicator, hence the dim apperance of this lamp. When on high beam the lamp gets full voltage and appears briter. The voltage now is "back feeding" through something else in the circuit.
