Experts say to put blankets on top of your child until the car warms up. You should be able to fit one finger between your child's shoulder blade and the upper strap.
"If you have a heavy coat, it keeps you from getting the harness tight enough. If the kid should take off their coat or in a big wreck he would slide out. It would not be tight enough to hold the child," states Kevin Theilen, Alliance.
Once you reach your destination, you can just put your child's coat back on in the back seat before you go out into the cold.