I hope this helps:
I live in Northern Nevada, in the High Desert, where summertime temps can and often do go over 100*F. Inside the car, without cracking a window and in direct sunlight, the sustained temps have regularly gone over 110*F and higher. I always have spare magazines in the car, in either the center console box or the glove box, and have never had any detonation issues.
I have noticed some degradation of performance of gunpowders (mostly in rifle calibers) when the ammo is hot to the touch, but nothing serious enough to result in malfunctions of the ammo or the weapon, even if the ammo is left in direct sunlight and is so hot to the touch that it is hard to load the magazines.
If it helps, Mythbusters did a series of experiments to debunk the myth of disposable cigarette lighters spontaneously detonating in a car heated by the sun. Myth busted.
Hope that helps.
Cheers!