My wifes Buick Rendezvous had the same problem when the defroster would be on for awhile. It went away for awhile but the odor returned last week. I noticed the Fuel Pressure Regulator was leaking. Its on the top of the engine where the fuel rail is. Its only held in by 1 bolt w/ and o-ring to seal. I loosened the screw and wiggled/jiggled the part then retighten the bolt. So far so good anyway