Heat is bad
I'm in Florida too bro, so I know exactly what you mean...anyway, I did some breif searches on various vitamins and supplements, and just about all of them said this:
Storage:
Store in cool, dry place away from direct light, but don't freeze.
Store safely out of reach of children.
Don't store in bathroom medicine cabinet. Heat and moisture may change action of supplement.
I've definitely always kept my stuff out of the bathroom medicine cabinet due to the heat/humidity, and given the heat and humidity down here, I can't imagine keeping them in a car would be any better. I was also an Account Executive forever on the road, I used to keep a days worth of stuff in a lunch box cooler, with an ice pack in the lid and a couple inside the box, worked great!