What is this part?

[msanta]

While looking for the turn signal bulbs i twisted this part(see pic below)and broke it. It was on top of a screw, before I broke it, I was able to move it up and down, like sort of an adjusting knob. It's right behind the head light case, next to the small orange light. There is one on each side of the truck. Can anyone give me some ideas?

 

[TechWrench]

I believe is is just a cap to cover the headlight aiming adjustment screws. If I remember correctly, it works like an adjustment lock, it keeps the adjustment from changing unless removed.