I'll definitely second the suggestion that it might have something to do with battery gasses......I went and looked at a couple SMB's before I bought mine, and one of them had a house battery with a cell that had gone bad. The smell filling the van was like something ridiculously organic-acrid, pungent and bitter/sweet and noxious.....kind of vinegar-ey......it was unbelievably repulsive.
At the time neither the seller (or I) had any idea what the smell was, he seemed pretty confused as to what could have been making the stink. A day later he called and let me know that he had indeed figured it out to be that house battery failing stupendously.
Not only horrendous stink, but it can't be great for your life expectancy to be inhaling much of that stuff.
Or --- as Herb (BroncoHauler) above mentioned too -- how about that mouse theory? Could some kind of critter have crawled into the dash/HVAC vents and died?
We always had to pull all sorts of mouse nests out of the cars we stored for the winter. Sometimes you'd find a really cute (but really sad) brood of mice-lings that hadn't survived the winter, but were still curled up, in a permanent slumber, in amongst all the seat-stuffings/twigs/fuzz that their parents had crafted a nest out of. Usually up against the heater core inside the ductwork.
Cute....but not sweet-smelling, to be sure.
'95 Ford E250 RB30 PH