An electric blanket is designed to run on a domestic 230V supply and it is unlikely that the 12V system in your car will provide enough power for it to work, and even if it did it would potentially drain the battery very quickly. You can buy inverters that take a 12V input from the car and provide a three-pin domestic socket, but we would advise checking the specification of your electric blanket first. Usefully, one of our readers has recommended a12V electric blanket designed for cars: www.ebay.co.uk/itm/165619089696