Mounting/Aiming Fog Lights?

Can anyone tell me how the fog lights mount on the steel (chrome) bumper? When I got my truck, it did not have the stock fog lights, so I bought some new ones. I know the housing has some clips that match the bumper, then an area in the back for a tab to keep it secure, but I don't understand how the actual fog light secures into the housing. The two nipples on each side go into the cutout for them, that much is obvious, but then the entire assembly just moves up and down and is loose enough to where they fall out when driving. Is a screw or something supposed to go into the bottom back, or am I missing something super obvious?

 

There is a slot in the back of the fog light housing that lines up with a hole on the truck for a bolt to go through. Just had the same issue. Stays tight once you put a nut and bolt on it.