Where does cabin air exit??

Hi guys. I need to figure out where the air inside the cockpit exits on a 550. I want to make some kind of device for constant ventilation of the cabin Even if the engine is not running. My trunk is also very hot inside and it is not ideal for storing things inside the trunk when the car is left outside and is running. My understanding is that when the engine is off, the hvac closes the intake of air? So obviously outside air need to meter and exit the cockpit... I plan on fitting a secondary battery with the benefits of bring able to ventilate the cockpit for houres without draining the main battery. It needs a switch between the batteries of course which I want to be able to manouver from within the car if extra amps is needed for either starting after s long period with the alarm on and the windows perfectly shut in the rubber seal. Also if the fans need to run constantly in stop and go traffic, an additional powerbank is not a bad thing as I don’t think the car charges more than it consumes. For example when it is very hot and max AC is needed. I need to fit the 575 fans (or stronger) to get the heat out of the engine compartment aswell. As the car is now, the doors need to be forced open if the battery is dead due to long period of alarm on. With a small hidden outside switch, parallell connecting the two batteries will solve this problem easily. This can also be great if the car needs to be left along the road if it breaks down because the lights and hazard will drain the main battery, but not both, if separated by a switch. Very very useful when left stranded in the middle of the night in far from civilazation places.

Any input?

Erik