Tailgate Open Switch

Remove the trunk trim.
(Refer to FRONT SEAT - "Trunk trim")
Check for continuity between the tailgate switch terminals.

Fig 1: Identifying Tailgate Switch Terminals
Courtesy of KIA MOTORS AMERICA, INC.
If continuity is not specified, inspect the switch

Fig 2: Tailgate Switch Continuity Reference Chart
Courtesy of KIA MOTORS AMERICA, INC.