This is the plastic piece that commonly breaks on Toyota Tacoma's that have locking fuel doors.
This is a very simple fix if you have a 3D printer. I used both a makerbot and ultrabot to print the part. The ultrabot had a finer resoultion so the part came out with a better fit. The best way would be to first add a washer on, and then the cap, then the lock. Super glueing the lock and cap. The washer isn't shown, but would be good to help prevent the plastic lock from catching on the metal housing.