Check for the continuity of the switch using an ohmmeter. If the continuity is not as specified, replace the switch.
Connect the motor terminals directly to the battery and check that the motor operates smoothly. Next reverse the polarity and check that the motor operates smoothly in the reverse direction. If the operation is abnormal, replace the motor.
Check the continuity between the terminals "3" and "5" with an ohmmeter.
Apply DC 12V to the terminal "3" and ground the terminal "5".
Check for continuity between the terminals "1" and "2".