Run it from a current limited switched mode power supply. Linear Tech and Nat Semi do many that will meet this requirement.
OR
Assuming a car when running is putting 14V out of the alternator, (14-10)/.9 is close to 4.7ohm as a preferred value resisor. Make it a 5Watt one on a small heatsink and you should be fine.
No need to worry about effiency as it's running from a car battery unless you have other ideas like leaving it on for a few weeks without using the car.