This pic clearly shows the part that is missing, are you sure its not still inside the tailgate, as I'm sure its larger than the drainage holes! it also appears that there is still something in the threaded part of the lock..., has the screw sheared off?
when you fix your problem. pay close attention to the T shaped lever arm, the shaft that they sit on can corrode, expanding the plastic, and making it susceptible to break very easily! Like most of these things, they will last forever if properly lubricated with white grease, and
not WD40 which will only wash away the proper grease!

Thom