Well M8 seems you may not know that when you turn your car off the cig lighter is OFF as well so you need to set your GPS to power down when there is no EXTERNAL power supplied. Easy peasy M8.
The only way to supply continuous power is to make a power outlet external to your Ignition switch as I have done in my Troopy coz I have GPS, Phone and Radio gear that needs to be on or charging while I have the Ignition OFF. You guys seem to be City folk and really haven't been out in the bush as we Ozzies say, so hey ya don't know/realize other things to do.
It only comes down to Experience and survival M8 so keep asking questions and we will keep giving answers.
I'm out in the bush/wilderness/outback /also places you have never been places yeh.
Da Iggs has been there.

Neil