They are on/off. The rotor wears thru the pad til the wires are exposed, then wears thru the wires' insulation, at which point they ground to the rotor and light the light up.

So accidentally grounding them anywhere else will light it up too. I'd unplug them and see if it still lights up, that'll tell you if it's the wires or the actual harness (that could be grounding too...) to them that's bad.