I notice you are from Phoenix too. My daughter has had a rash for the last week. I took her to the doctor who said not to worry about it as she didn't have a fever but I wonder if it could be heat rash. I didn't really think of it because it has been cooler here over the last week. Does the rash bother your LO?
DS has been getting one around his diaper area. I let him have naked time (watched carefully for pee, LOL) and sometimes I use disposables because the CD's seem like they retain heat more.
We are in FL and experiencing 100+ degree weather, too! My DD had heat rash and we pretty much left it alone and it went away. Obviously, dressing your LO in lightweight cotton clothing helps, and I found that even though my car has very dark tint, it helped to add those window pull shades in the car (like this one: https://www.amazon.com/Fisher-Price-Ocean-Wonders-Shades/dp/B000M9GC88/ref=sr_1_24?ie=UTF8&s=baby-products&qid=1280746664&sr=1-24). Her carseat is black, so I added a lightweight cotton blanket in white and that seemed to work, too. We haven't had heat rash since....
Re: heat rash