Your auto can't get hot enough for ammo to ignite, it's perfectly safe in there.
It may mess with the effectiveness of the gunpowder after a very long period of exposure. The biggest problem ammo has with changes in heat is the condensation. Since you're in AZ, humidity isn't going to be a problem like it would if you were lets say in NOLA in the middle of August.
If one of your guns is stored in a car the question is -
Is it secured while it's in the vehicle or tucked under teh front seat?
You could store it in a locking insulated safe. Honeywell makes a few models that will fit into a trunk or under a truck seat. (available at Lowe's and Home Depot starting at $30 or so.
Just about anyplace you can store it in your vehicle, and insulate it from additional heat is all the better. Keeping it in an unprotected or uninsulated place will also make the gun too hot to handle on some days. Ever burned youself on a hot soda can? Imagine a gun in your hands in the "heat of the moment" please pardon the unintentional pun.
I keep mine in a safe in the trunk of my car when I need to leave it there.
I have a Honeywell #2017 safe like this one (12.8 x 16.1 x 5.8"):
($60.00 +/-) Fire and Waterproof
you could get the #2013 (11.1"X13.9"X6.1"):
http://www.honeywellstore.com/security- ... safes/2013
($40.00 +/-) Fire and Waterproof
you could get the #2011 (11.4"X13.9"X6.1"):
http://www.honeywellstore.com/security- ... safes/2011
($30.00 +/-) Fireproof only
The least expensive place I've found for these is Ebay, even after the shipping is added, but a local store is much more convenient and faster to get one.
Just some ideas.