It gets about the same around here, and I've never had an issue with heat. Generally by the time the carpc warms itself up enough for it to be an issue, the car itself has gotten down to a temperature where it's not that hot inside. A trunk mounted system would not have the benefit of your in-vehicle air conditioning or an open window, but the trunk also does not get nearly as hot as the interior of a vehicle.
I've seen people duct fans from the cabin back to the trunk before, which could help in reducing ambient temperatures around the pc. I think the main problem you would run into with the heat would be processor temperatures (With a SSD I wouldn't worry about HDD temps). You can easily figure out how hot your processor will run by finding out how hot it runs inside your house. Temperature differences for the most part stay the same in the small range that we're talking about. If your processor runs 30C over ambient in the house, it will run 30C over ambient in the car (Even if in the car it's much hotter). Figure out what is an acceptable ambient temperature for your trunk, and add ducting, or some other ventalation to help until you bring it down to that level.