I had the same problem with our 96 ES300. Would you believe it was the windshield wipers? After removing the parts to get into the vent area under the hood, spraying everything with Lysol, and so on, while reinstalling the wipers I noticed the smell on the wiper blades, which were about a month old 'ANCO Contour'. I returned them to the store, installed a different brand, and it's been perfect ever since. I live in Minnesota and summer or winter never had the problem before or after the wiper replacement.
I read a story similar to this, and didn't believe it. Check it out, it might fix your problem too.
