I drive a 2006 Toyota Sienna. When driving with the heater on I often smell sulfur. This is more common when I am driving up a hill, but if I turn the heater off, the smell goes away. What causes this smell and how can I repair it?
When you turn the heater off, are you turning the thermostat down, turning the vent fan off or both? Are you closing the vents? Do you smell the same thing with the AC on?
Depending on the answers, here are some possibilities:
Following up on the comment: are the side vents passive only? Are they taking in air from the same channel? Also, when you run the AC, are you running on recirculated cabin air or taking in air from the outside?
As far as leaks go, you could try putting newspaper under the car when you park it. That will likely expose a drip that would otherwise go unnoticed.