Okay, then. I would recommend simply bypassing the power switch on the side (solder a jumper wire across the terminals) and routing the power wires through, for example, the radio circuit (power to the radio turns on when you turn the ignition switch). By bypassing the power switch, it will be on any time there is power to the navigation system, and it will shut off when you cut the power (turn off the car). I think this would be one of the easiest ways to do what you want to do.
Der Strom