All plastics leach into liquids stored in them...the cheapest all the way up to lexan. Heat and cold synergize this effect. Even the clear plastic soda bottles are permeable. That's why soda in plastic will go flat long before that in cans and bottles.
My experience regarding the POTABILITY of water in plastic exposed to temperature extremes (as you'd get in a car) is similar to Susan's experience outlined above. So, while I do not carry liquids in the car (except what I carry daily), her statement:
Difficult choice: drink plastic water or die of dehydration?
does have a certain amount of truth in it. It is an acute vs. chronic choice. If I have no other way of getting liquids in me, and I'm in a likely survival situation, the choice is obvious.
However, in most cases when I reach for emergency preps, I am not really in dire circumstance...I'm more acting out of readiness and immediate need. Thus, band-aids, tools, etc get used from my packouts. I carry 1 liter Nalgene lexan bottles with liquid (usually water or tea) almost every waking moment...sometimes several. But I use them, refill, move on...pretty much like many packout materials.
Ah...the old days when you could get 5 gallon glass water bottles...sigh. Not that it would help in a car situation...