That's a good idea for cheap, are there cup holders specially designed for ecigs?
Yep! There's a lot of them, and people are actually 3D printing their own and selling them for super cheap (check Etsy and other crafts sites).
But to answer the initial question, I avoid leaving my mod in my car, particularly with the battery in it. Cold AND hot can have negative effects on metal/Lithium cells. You wanna be careful about this. So when I go to class or something like that, I usually toss my mod in my backpack, even though I'm probably not gonna use it until I get back to my car. I just don't want to risk the metal shrinking, or the battery getting too hot, or anything like that.
Also juice. I don't leave juice in my car. The cold probably won't affect it at all, but the heat will DEFINITELY make a difference. I've heard of people accidentally leaving juice in their car for a day or two (in the heat) and the juice being totally ruined as a result. It wasn't even in direct sunlight, just in a car w/ closed windows in 90 degree weather. Imagine if it was the entirety of their juice collection. That'd be a lot of money down the drain.