If you are camping on the south rim of The Grand Canyon the altitude is about 7000' while the north rim is closer to 8000'. Summer high temperatures in northern Arizona at these altitudes are not likely to exceed 80 degrees. Car interiors can certainly be hotter than this, though, if they are parked in the sun. I have repeatedly left gas canisters in my car while parked on the south rim with no problems but I make a point of parking in mid-day shade. Personally, I don't worry too much about storing gas cartridges under these conditions but my experience is anecdotal so take it with a grain of salt. It would certainly be easy enough to hide the canister(s) in a shady place outside of the car or to decide to use a white gas stove instead of using compressed gas.
Are you talking about using one of the Camping Gaz stoves, the ones where you can't separate the canister from the stove until it's empty? If so, wherever you put it, ensure it won't move or get jostled and inadvertently turn on the gas.