As Glenmore notes, all these do is provide a grounding path for the light bulb. When the pad wears to the point that the rotor chafes off the insulation on the wire, the circuit is completed and the bulb lights up. Any point of ground contact will also light it up, so you'll need to check them all to locate the point(s) of contact, assuming the pads aren't yet worn that far..
I've been successful in reusing the front ones by dabbing the wire ends in RTV sealant, to 're-insuate', before inserting into the new pads.
