When using the defroster, a chemical like odor emits from the vents and I and I can eventually I can taste a bitter taste on my lips and irritation to my nose. Too strong to use and I have to turn off the defroster.
Have you had any coolant loss? I remember one guy who had a pack of matches fall into the defroster vent, and had his dashboard catch on fire.
The dealer thinks there’s a “kink” in the exhaust system due to a minor bump to the rear bumper (which occurred 2 years ago) and no one else can smell the odor.
An exhaust system should not be hard to check for leaks, perhaps a small amount of oil is leaking onto the engine, most likely the head gaskets. or http://community.cartalk.com/posts/list/2191392.page