I ran into the same problem not long ago, so I did some research. Turns out that the biggest issue in that situation is your windows. Windows let hot sunlight into the car, which heats up the air inside. However, since the air is trapped inside the car, that heat has nowhere to go. It starts to build up inside. That’s the reason why a closed car is so much hotter than its surroundings. In fact, using glass to let the sun into a closed space is the same principle that allows greenhouses to operate.
You can solve that problem in three ways. First, you can hire a window tinting Wheat Ridge company to darken your windows. The darker they are, the better they’ll be at blocking out the heat. Second, you can block your windows with an opaque material while parked. A reflective windshield shade works best for this.
The third solution is to crack open your windows before leaving. Doing so allows more of the hot air to escape, slowing down the greenhouse effect. The issue, of course, is that a gap in the window makes it easier to steal your car. This method is only practical if you parked somewhere safe or somewhere you can watch the car.