"$g_presentation_obj_admin_panel_6" is connected with the event multiplayer_event_admin_set_respawn_period. It has however no direct influence on the respawn time in the Siege mode. So the only other option is that it's inside the siege mission template, "multiplayer_sg". And there you will find multiplayer_siege_mod_defender_team_extra_respawn_time. Now look up everything which is connected with that.
In general: If you are searching for something, try searching at places which can be connected with it. In this case one option was module_presentations with the admin menu (a wrong guess of mine) and module_mission_templates and there simply the siege mission template for multiplayer.