I woulda thought that the trunk is the hottest part of the car
It doesn't have windows, so it doesn't get as hot. Additionally, when it's packed with stuff, the stuff can absorb a lot of heat, so it takes a while for the ambient temp to rise. I once did it for a good few hours in Lancaster on a hot sunny day, I kept going back to make sure my teffilin don't get too hot, and was shocked by how cool it was. An empty trunk would get hot fairly quickly.