Something does not sound right and I am unsure who is not telling you the truth. Contact the internal affairs of the police department and explain that a) your vehicle was taken without notification to you; b) that it is your vehicle and he is merely a co-signer on the loan; c) you understand he detective drove your vehicle which is against protocol and d) you will file suit if your vehicle is not returned to you. Then, contact as many politicians who have a history of being critical of the police department and file complaints through them and then sit back and watch what happens. If nothing happens, you should immediatly contact a lawyer to get your car back and sue the police department for your expenses.