Installing a CB unit and want to mount the antenna left side rear and existing taillights were in the way of where I want to put mounting hardware for antenna. As stylist as the taillights are (

) the were in the way and gave me a good excuse to install LED taillights.
Had to lower the license plate holder and one of the mounting holes in license holder involves drilling into a metal box that appears to go the length (width) of tub. Can't tell whats in the box, if anything - may just be reinforcing.
Anybody have a clue what its for. Didn't measure but its something like 3" square. 2001 TJ. Maybe ought to start another thread so someone sees this.