If you have to leave your car to build a snow shelter, leave a note in the car telling road crews and/or EMS personnel that you did so and are still nearby. That way they know to look for you. In that case, I'd try and make your shelter very noticeable, e.g.: colored tape to tie around the trees nearby, a bright plastic flag stuck in the ground by the shelter, aluminum pie pans that will swing in the breeze, etc. Anything that will draw attention to your shelter. Hell, if you're not that far, you could tie a rope (or your extension cord) to your car and tell the EMS crew to follow it up to the door of your shelter.