The actual tailgate dome light itself sits very similar to the cabin dome light. (identical I think) there is no other switch behind, below or any where around it. Maybe the switch of your diagram is referring to the "ON" ( steady on) & OFF "15 sec delay" manual switch on the physical light itself.

When I dug into my Tailgate today I noticed something that kind of has me curious to what ANDREW is mentioning. When I would lightly push in on the lock/latch thing on the tailgate the light would come ON when pressed and OFF when released.. Something around here or near the rear wiper has to be "retarded" as it is working perfect except for the command is backwards. ON when closed OFF when opened... UMMMMMMMMM