No chance of it being the heater core?
Side note: A bud of mine had his windshield replaced like... 5-6 years ago on a Ford Escort, it would weep from the seal at the top cuz shitty install. It wasn't noticeable at all. No leak in car washes, or during rain. But over night, and during rain/snow it would accumulate while sitting.
How he finally figured it out was parking on an incline which made it drip down the inside of the windshield.
Could be a thing to look out for. Also, I share feels with ya on the smell...it sticks around unless you root it out, which is quite a job.