From what you wrote, you want an independant 12v pack. I would use 10 ni-mh "AA" size batteries. Energizers from Walmart are usually 2.1 AH.
You can connect the power to the 12v outlet plug wires. The current is cut while the key is not in. You add a diode to prevent the custom ni-mh pack to discharge into the 12V system and let the 12V system charge it as needed.
You can use like the previous tread, computer style fans. What I am not aware of is where to put the fan. It needs to either draw air into the cabin or move it out.
You can connect the power to the 12v outlet plug wires. The current is cut while the key is not in. You add a diode to prevent the custom ni-mh pack to discharge into the 12V system and let the 12V system charge it as needed.
You can use like the previous tread, computer style fans. What I am not aware of is where to put the fan. It needs to either draw air into the cabin or move it out.