If the wire has worn insulation at the end (where it inserts into the pad), the warning light will come on on the dash. The friction from the disk/rotor wears the insulation off when the pad is worn down enough, grounding the bulb to illuminate so you'll know the pads need replacing. I dip the worn end in a bit of silicone or that liquid tape stuff and re-use the wires.

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1986 560SL
2002 Toyota Camry
1993 Lexus