The co-signor on the loan that you have where you have the main repsonsibility for making payments on it does not have the contractual or legal right to report the vehicle stolen if in fact it was not unless you have a separate agreement in writing allowing such. Likewise, the co-signor does not have the right to sell the car without the primary borrower's consent and the consent of the lender.If you take the car that you are making payments on where you are the person primarily obligated under the loan for it, there is no theft of the car from a legal perspective.The obligation of the co-signor lies lies with the lender where if you fail to nmake payments on the loan and the primary borrower, the co-signor then is required to make up the payments. If that happens, the co-signor can then look at you for repayment with accrued interest.