You slide the "cage" into the dash opening, then the little tabs bend over, to the back side of the dash, to hold the cage.
The radio then slides and locks in, usually there is a stud or screw hole in the back side of the radio to hook a brace up to support the back of the radio.
Then the cover snaps on, to hide the "cage".
To remove the radio, you then have to remove the cover, and use the two supplied thin metal keys slide down the side of the radio to release it.
