A door operator assembly is configured to raise and lower a cargo door of a cargo enclosure. The assembly includes a power and control unit mounted on an exterior of the cargo enclosure. The power and control unit includes a motor, a coupler operatively connecting the motor to a torsion assembly of the cargo door, a manual release mechanism for actuating the coupler to connect and disconnect the motor and the torsion assembly of the cargo door, and a control circuit for the motor. The assembly may also include an operating switch disposed on the exterior of the cargo enclosure.