Locate the door hinge pin in between the two molded bumps on either side of the backplate.
Pull out the hinge pin with pliers. This will expose a latch on the backplate.
Open the latch and slide the metal door on the unit to the open position.
Place the rear of the TLR light into the open doorway and slide the latch back into place to lock it.
Insert the pin into the unit with the pliers. Once the pin is set, the backplate will be properly installed.