The motor case does not serve as a grounding connection, nor is it designed to carry any current at all.
There are two wires entering the motor, neither of which show continuity to the motor case.
Through the various relays and limit switches the polarity of the two motor wires is changed. A polarity change on a DC motor changes the direction of rotation for that motor.
Near the center of the diagram, BK denotes the grounding connection for the relays and for the motor.