This makes more sense with AppleWatch. If your phone unlocks and starts your car just based on proximity...problems. TouchID would be more secure but less convenient. AppleWatch + Siri would be best.
It is unlikely you will loose your watch. You can just ask Siri to start the car or open the trunk when you ACTUALLY want it not when it THINKS you do.
When you are walking toward your vehicle you could get an alert asking if you like to warm/cool the vehicle.