Without Power Tailgate

Open the tailgate.
Remove the tailgate trim panel ( TRIM REMOVAL/INSTALLATION - TAILGATE AREA ).
Disconnect the 3P connector from the actuator.

Fig 1: Identifying Tailgate Lock Knob Switch 3P Connector (Without Power Tailgate)
Courtesy of AMERICAN HONDA MOTOR CO., INC.
Check for continuity between the terminals. There should be continuity between the No. 2 and No. 3 terminals when the tailgate lock knob switch in the UNLOCK position and no continuity when the switch is in the LOCK position.
If the continuity is not as specified, replace the tailgate lock actuator.