Thanks Brad but what a dopey idea. My 1988 Mercedes W124 has clip-in wear sensors on both front brakes (can't remember about rears). They snap into the inside pad and have a short coiled lead to the permanent cable which follows the brake hose. They work by earthing to the disc/rotor. Useable many times over provided you change pads promptly on warning and you don't subject your brakes to race conditions.

