To combat this problem, we put a lighter, hooded jacket on under the heavy coat, took off the heavy coat in the car, and put a blanket over the child. We leave the blanket in our car during the winter months. Between the blanket, a hood or hat on their head, and the heat in the car, our kids have stayed warm. The blanket is nice because if the child gets hot, they can just take it off. The light jacket is helpful because they can still keep it on indoors if they are a bit chilly.
We have car seats that go up to 100 lbs. (they convert to high backed booster~our son stayed in 5 pt. harness until he was 56 lbs. then went to high backed booster), but we still use the blanket due to safety. And, quite frankly, it is a pain in the butt to have to constantly adjust straps! Good luck!