Be careful with a plug-in cooler. Friends took one on a trip from SoCal to Las Vegas. They left it plugged in the car all night and got up to a dead battery the next morning.
I've been using one for probably 30 years, But you are right. No switched power outlet on a 70 Dart. I drove with 3 kids in a 73 Dart for a number of years from MD to SD every summer to see their Grandfather.