I've seen systems fail under the weather, but for different reasons. A laptop's most critical (and warmest) components can easily reach 60 °C above the environment temperature. They're designed to withstand this torture, because notebooks don't usually leave a lot of room for efficient cooling, nor can they afford to be as power consuming as other systems. When the room temperature goes up, so do the temperatures of the processors.
Today, overheating seldom damages the system, because all modern laptops have a number of precautions. The temperature is monitored and the cooling fans react accordingly. If they can't transport enough heat, the CPU is clocked down to reduce heat dissipation. If the temperature gets dangerously high, the computer shuts down altogether. Different models have different shutdown and maximum temperatures, but they're typically between 90 °C and 105 °C.
A laptop left in a car being powered down wouldn't reach those temperatures if it sat on the engine, let alone on the back seat. You absolutely needn't worry about the electronics breaking down, but if the notebook is left in the sun, it may get hot enough to compromise the structural integrity of the case, which is a fancy way of saying the plastic will get sticky and smell funny. If it does, don't sit on it until it has cooled down. Other than that, you're in the clear.