Let's say you were going groundhog hunting after work and left your ammo in the car all day so you could leave right away to go hunt. It has been in the high 90's here and I was wondering how hot it would have to get in a vehicle befor the ammo would explode.

Won't happen, you need several hundred degrees.  Your seat will melt before the ammo cooks off.

The short answer is you have absolutely nothing to worry about.

The longer answer breaks down to what would 'cook off'.  The primer powder is much more temperature stable than the smokeless powder, and it will not ignite/explode from the very low (relatively) temperatures found inside a car (<170 F).  Over time, the primer composition will break down, but we're talking many years.

The smokeless powder makes a more interesting topic.  Its stability is something that was studied extensively in the decades after smokeless powder was first invented (1864).  Smokeless powder will ignite at several hundred degrees Fahrenheit, so nothing to worry about inside a car.  But it has a predictable decomposition rate at a specific temperature.  Again, it would take years even at the temperatures inside a car in the middle of summer to cause appreciable decomposition.

You can find a complete overview of this topic in the smokeless powder chapter (p.287-330) in Tenney L. Davis' book "The Chemistry of Powder And Explosives", available from Amazon.com.