2004 Toyota Prius Owners Manual - Page 123

The system will automatically shut off after the defogger has operated about 15 minutes. CAUTION Since the mirror surface can get hot, do not touch them when the switch is on. Make sure you turn the defogger off when the window is clear. Leaving the defogger on for a long time could cause the auxiliary battery to discharge. The defogger is not designed for drying rain water or for melting snow. If the outside rear view mirrors are heavily coated with ice, use a spray de−icer before operating the switch. NOTICE When cleaning the inside of the rear window, be careful not to scratch or damage the heater wires or connectors. 111