Typically, a co-signer is someone who agrees to repay a loan if the primary borrower defaults on the payments. This means that the co-signer is legally obligated to make the payments if the primary borrower does not. However, this does not mean that the co-signer has any ownership rights to the vehicle.
If the primary borrower defaults on the loan and the co-signer makes the payments, the co-signer may be able to take legal action to recover the money they paid from the primary borrower. In some cases, the co-signer may also be able to repossess the vehicle, but this would typically require a court order.