Comment Use a switch and some relays (Score 1) 72
What is needed is some relays in series with the car's electrical systems and a switch or switches to control the relays.
When you park the car, use the switches to de-energise the relays and thus electrically isolate the car's electronic systems ( this will also handily stop the engine ).
When you want to use the car again, use the switches to enable the relays and thus power the car's electronic systems.