Long story alert. The weirdest electrical issue I ever troubleshot involved a set of Cessna Caravan airspeed switches. The airspeed switch that controls the BELOW ICING MIN SPEED annunciator burned out, so we replaced it. Promptly fails again. Install a second one, notice that it ops checks ok while hanging by the wire harness and pitot/static lines, but as soon as we bolt it in place, it fails (accompanied by magic smoke smell) and nothing will bring it back.
Ok, clearly something is going on here. Since the problem comes when it’s installed, I check the mounting bracket for voltage. Sure enough, the mounting bracket is showing 24 VDC. At that point I go through pulling circuit breakers until the voltage goes away. The circuit breaker that fixed it was the Overspeed warning, which has another airspeed switch mounted to the same bracket.
Turns out the overspeed switch had shorted internally and was sending power straight to ground through the case. But, since the bracket was a later installation (part of the Caravan icing ADs), and had been apparently very thoroughly painted before installation, isolating it from aircraft ground, it wasn’t tripping the breaker. So the icing airspeed switch was providing a path to ground all backwards, burning it out in the process. 2 new airspeed switches, and a little work with sandpaper to properly ground the bracket, and we were back in business.