Put a My Carseat Blankie, under a car seat cover that does NOT go behind the baby. But make sure you place it on after you buckle the harness. My sister found this wonderful car seat cover is made by Soothe Time™ it just wraps around the top of the car seat. It has a full zip on the bottom and the top. And it has a water resistant outer shell and a soft flannel inside! She uses it with my nephews My Carseat Blankie all winter long.
I've seen a few others at BabiesRUs from J.J.Cole, and Especially For Baby. As long as it's just attaching to to the top and not going behind the baby you are using a safe solution.
The Safe Solution: Car Seat Safety Experts will tell you NOTHING should ever be behind a baby or between the harness, just strap them in and then bundle them up.
You can view what different experts have to say here: