While taking the ferry to Colonia, the dock inspectors told me an Arg.-tagged car could only be taken out of the country with a DNI showing Arg. residence. An Argentine passport or a cedula are not enough - they have no addresses.
That was last year - things may be different now. Those regulations change all the time, and they are not always enforced. Once my car was overlooked and not checked at all, and another time I did not have the exact right papers but the inspector liked my dog and let us board anyway.