You could run some wiring direct through a hole that you could drill in your floor panel to your battery, that's not too bad of an idea. I used to run a wiring on my old car from my fan (to make it work), to one of my heating options that I never used. It pretty much meant that if I pressed the button, my fan was on, and my car didn't overheat. It was a pain if you forgot though, but forgetting doesn't matter for a fridge, except for your drink being warm (hardly gonna matter, it's not even beer (dont drink & drive))