One problem related to defogger heater has been reported for the 2004 Toyota Sienna. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Toyota Sienna based on all problems reported for the 2004 Sienna.
In winter time, only rear vents blow cold air even if the rear thermostat switch on red color. Dealer inspected and said that is normal. Leave the van running for 25 to 40 minutes until get warm air. All last winter I had to turn off rear vents.