One problem related to defogger heater has been reported for the 2003 Toyota Highlander. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2003 Toyota Highlander based on all problems reported for the 2003 Highlander.
Heat stopped working, blowing cold air. The problem is the ribbon wire from the temperature control knob to the electronic board behind the switch breaks, this is caused from the knob stem loosening, eventually breaking the wire. Dealer will replace the unit for $900 found many people with this problem on tundrasolutions and found instructions to remove the unit and solder the wires, works like new now. Seems like this problem may also be the root cause of the dashboard fires listed on this site.