Funny, I have a pair of Bolle sunglasses that have taken up permanent residence in my car. Sometimes in their case, in the glove compartment, but just as often, out in the open, uncovered, in the sun. I've left them that way for days at a time in the heat and I've never had them warp, bend, disfigure, or otherwise change in any way due to the heat in the car.

I keep most all my most-often-used fishing stuff in my trunk (3-4 rods & reels, 2 vests, waders, boots, etc.), and have never had a problem with it.

Your mileage may vary, though, of course. Just try leaving some gear in there for a few days at a time in the heat, making sure to check on it when you think of it. If it's fine after a few weeks of 90s temps, you're probably good to leave it in there.