looks like my heatercore in the 88 decided to leak alittle. time to change it out soon, yay! guess could be a good time for a writeup on doing it tho :P pix/ guide to come as i get time to do.
You interior is wet from raining this week and temp is dropping fast overnight. Many cars are have same issue. My Kia had frost inside last week.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Yesterday morning my Grand Prix had ice buildup on the inside of the windshield, I had to scrape it from the inside. Might want to hold off on the replacement!
I know exactly what the OP is talking about. It's got nothing to do with moisture leaking into the car or temperatures dropping or whatever.
When the heater core starts leaking, which mine is now doing , air going through the heater core picks up this oily moisture and it condenses on the inside of the windshield. So when the defogger is then used to clear the windshield, it actually gets worse!
Fortunately the heater core in a Fiero is relatively easy to replace, but it's still a nuisance.
Here's something interesting on replacement heater core prices - A local place that I don't usually deal with (because their prices normally suck), has two very different prices for Fiero heater cores depending on whether they're for an A/C or non A/C application.
In fact I think GM, even though probably unintentional, made a smart move by making a heater core so accessible and the change to take about 15 minutes vs. hours with other cars like my GMC Typhoon (pain in the *SS). Mine took 30 mins but that's just because of rotting stuck rubber hoses. Probably the easiest thing I have changed on my fiero besides maintenance things
------------------ 1988 Pontiac Fiero Gt (fully optioned, factory leather, 5-speed, CJB T-Tops: Black w/ gray interior and blk wheels) 1992 GMC Typhoon #0203 1971Chevorlet Corvette 454cu/ LS5/ 4-speed/ convertible 1993 BMW 850 Ci 6-speed V-12
I had another fiero at one time and would park it outside, the same thing happened with the dew on the inside of the windshield. I found out my windshield wasn't %100 sealed as soon as I found the break, I sealed it up good. and after that no problems!
if the heater core was leaky it would smell like coolant in the cockpit. XD
or maybe the temp change was so fast the interior temp didn't change fast enough and so condensation formed.
kinda sounds like a party, quite interesting though; as to how the coolant condensed onto the windshield after evaporating.
seen it happen a lot to our work trucks, not sure how/why, just does, ill see if I can get a pic of it tomorrow morning if im not in toomuch of a coffeeless zombie state.
kinda sounds like a party, quite interesting though; as to how the coolant condensed onto the windshield after evaporating.
Hot cooling system under pressure is over the boiling point, but still a liquid....as it leaks the pressure drops: it quickly turns into vapor inside the car and will condense back to a liquid on the first cold thing it touches. This is normal stuff in snowy climates.
, air going through the heater core picks up this oily moisture and it condenses on the inside of the windshield. So when the defogger is then used to clear the windshield, it actually gets worse! 
