Just a guess since I don't have a VIA EPIA-M board, but most modern PC motherboards have built-in temperature sensors (commonly referred to the CPU and case temperature). BIOS's are usually very liberal on determining if a system will boot-up at a certain temperature. The act of opening your car door after it has been sealed and in the sun will drop the temperature considerably.
There's almost no way around this problem other than finding a cool airflow to your C134 case while you're away. So I myself am not sure what else to recommend other than telling the BIOS *not* to automatically shut the system off when the temperature is over a certain range provided that the system cools down quickly after you boot it up.