A warning triggered by a substantial electrical load gives a useful test condition. It does not prove the demister itself or the body controller is the failed part.
Record simultaneous changes
Note dimming lights, engine-speed changes, dashboard resets and whether other high-load functions cause similar symptoms. Include charging-system and rear-window work. Do not repeatedly run a circuit that smells hot or behaves abnormally.
The workshop should compare command, load and supply behaviour using the correct procedure. A normal voltage reading with the demister off may miss the problem. Provide the exact trigger and measurements so the investigation can distinguish a local circuit defect from a broader electrical supply issue.
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