This is by no means scientific or a recommendation that anyone else do this, however it may put some minds at ease a bit.
I live in Tucson, Arizona - one of the sunniest and hottest places on the planet. I leave my pickup truck parked outside my office all day in direct sunlight with the windows rolled up. The temperature outside reaches 110+ degrees in the summer. Inside, it must be 130-150 or so for hours on end, day after day, all summer. At any given time, I have between 10 and 50 CDs in my truck - some pressed and some burned. I sometimes have DVDs in there too. I make no special efforts to keep the disks out of direct sunlight. (I don't leave them out on the dashboard either - just on the seat where they usually get a couple hours of direct sunlight a day. Some of them in jewel cases and some just naked.) I have never had any heat related problems whatsoever with any of the CDs or DVDs I leave in the truck. I have had some jewel cases melt on me, but I've never had any disks damaged.
My conclusion: No natuarally occuring condition on the planet Earth (with the possible exception of a volcanic eruption) is hot enough to adversly affect CDs or DVDs. This is nothing to worry about. This is just my opinion and is based on absolutely nothing by my personal experience. I take no responsiblity if your CD melts in your car.