When I nurse in the car (I loathe pumping and would rather show my boobs to everyone in the world) I sit in the backseat, stick a blanket in the window (like roll it up in there), stick one from headrest to headrest to block the front and make sure the carseat canopy is up! It's like my own little nursing cave.
We have an in car blanket that is always there and I use a receiving blanket I always carry for the headrests.