While a co-signer's name appears on a loan agreement, the title of the car is still in the name of a person who purchased the car. The title defines ownership, so the co-signer has no right to seize the vehicle from the owner.
A co-signer is not a co-owner of the car. In fact, a co-signer has no rights to the car, only financial obligations. The owner of the car is the only person who is able to possess or sell the vehicle. If the payments for the car are not made, the car is repossessed and sold, and the co-signer is responsible for paying the difference between the money made through the sale and the money owed for the car by the previous owner.