Turn it over to your insurance company; if you don't have insurance, then contact the insurance company that has sent you the demand letter, and provide them with a copy of your police report, or at least let them know about your car being stolen.
Contact your insurance company immediately. You have the police report. Hopefully you had already reported the car stolen before it was involved in the collision. By contract, you insurance company is obligated to defend you, but not necessarily for a crime such as a hit and run. Marshall all the evidence you can that you were elsewhere when the stolen car was involved in the collision, including witnesses, telephone land line records and so on.
Turn the case over to your insurance company. Best of luck.
This answer is provided by Manuel A. Juarez, Esq., El Abogado de Accidentes de Autos de California: 510-206-4492. El Abogado de Accidentes de Autos provides answers of a general context. These answers are not intended to form an attorney client relationship. El abogado de Accidentes de Autos y Lesiones Personales practices in Antioch, Berkeley, Concord, Oakland, Hayward, Martinez, Newark, Richmond, San Francisco and San Rafael. El abogado de accidentes y lesiones is licensed only in California. This information is good only in California and it is not to be taken as legal advice on car accidents, personal injury, divorce, bankruptcy or in any other type of situation. Esta respuesta es del Abogado de Accidentes de Autos y Lesiones Personales de Acidentes de carros, Manuel A. Juárez, 510-206-4492. Abogado Hispano de Accidentes, Divorcios, Abogado Latino de Accidentes, y Abogado de Acidentes de Oakland, Hayward, San Francisco, y California. Estas respuesta son solo para información general y no consisten en consejo legal sobre divorcios, mantención de esposas, mantención de hijos o bancarrotas. Las respuestas son comentarios legales que no forman una relación de abogado y cliente. Manuel Juarez, Esq., esta licenciado solo en el Estado de California.