...well.... at least you'd feel cold as they carted you into the ER with freeze burns on your nether parts.
Refreezable packs could replace ice but I still stand by desiccant agents to get the moisture out of the air. Also prevention of direct sun hitting the glass in the first place will go a long way and making cardboard shades custom fit for the windows and attaching them OUTSIDE of the car with magnets or just blue tape will go a long way to blocking out those pesky infrared waves. (more efficient than in-car behind the window shades since the energetic waves have penetrated the glass and the energy is trapped in the car)
Looks cheap but consider it more theft deterrance.
Also, if you park in the 6th level of heck, I wouldn't leave the vents open. I find A LOT of heat from the outside matriculating in through them to warm up what cool air is left in the cabin. But if your car color is dark ... well.... you're screwed.