Burning smell when front defogger is engaged

[Pianodemon]

I've noticed a pretty nasty "burning" smell within the last few months whenever I turn on the front defogger in my '98 Pathfinder.

It doesn't happen with the main vents, just the defroster.

Any ideas? Leaf buildup somewhere that's decaying and filling the car with funk?

[audtatious]

Doubtful unless you notice a difference when switching between recirculated and outside air sources. There are some spray cleaners which you can shoot into the duct work to clean them out but the name escapes me. Trying something easy like Lysol spray disinfectant may help?