vega480, I live in Mesa, too (when I'm not working in Iraq).
I once went through a period where my employer did not allow weapons on the property, including the parking lot, so I had to be sneaky with mine.
I kept my pistol in the trunk and never had any trouble. It would be a little to hot to actually hold in my hand by the end of the work day, but it cooled off on the ride home.
I would be leery of keeping it in the car, unless you keep it OUT of direct sunlight. I have actually had a 3.5" floppy disc MELT on the back seat of my car. I don't know what the cookoff temp is for ammunition, but I don't want to find out by experimentation, either.