Can anyone identify this part for me?

Can you tell us more about this piece? Is it a thumb screw or something else? Does it unscrew completely, is it easy or hard to screw in?

what is the brass screw in the photograph? It looks like the box has holes near its bottom, did the brass screw come from one of those holes? Does the top lift off of the base with the screws removed?

sorry I only have questions, I can  not identify the box.

Larry

What does it look like underneath?

I apologize.  That free screw is only one of the two screw which held the box to the trunk wall and not one that fit into the box in any way.  The box is sealed all around with the back being aluminum I assume (as it is not at all rusted).  There’s no way into the box and the brass thumb screw on top comes out pretty easily …and has little to no resistance going into the hole either.  Shaking the box lightly produces a drum like sound which I suspect is a capacitor or resistor of some sort.  

The single wire is odd. I suspect some sort of filtering device for a power supply. Definitely no longer required.

Agreed.  I’m clearing out all the old wiring as best I can.  I had the passenger side door courtesy-light (the one on the door jam side of the door itself) working fine, but after pulling out seemingly unrelated wires it no longer works.  Frustrating as all hell.