Have you ever had problems with your endgate window washer?
If so, someone may have removed the trim panel. When this is done you have to be careful to disconnect that lamp. Not to say that it's broken but maybe someone got in a hurry to reassemble it and it's just not plugged in.
The way it's wired, if all the other interior lights work, a fuse is not your problem. They are all turned on by a single relay that you have to remove the second row seats to get to. You don't even want to try that.... trust me.
Getting the endgate panel off is pretty easy but putting it back on is a little tricky, if you want to try it let me know and I'll give you some tips.
Otherwise if it was serviced, you could take it back and tell them since they worked on it the lamp doesn't work.
That way if the broke it, they can pay to fix it instead of you taking a broken part in and them telling you that you broke it.