it sounds like it might be in a car doesnt it (engine frame??)
cars and vans use a negative earth setup, so the feed goes to the cabinet thing from "+" on the battery (via a fuse), and the neutral (or commonly called earth in automotive setups) ends up going back to the "-" terminal on the battery, usually via a metalic part of the chassis. its not normal to attach straight on to the negative on the battery.
to earth the cabinet? trying to think about this logically....if you connect cabinet to car chassis then that connection ends up going back to the "-" on the battery, as well as the main power for whatever your running, not hundred percent sure but if yuou connect it like this then your making the cabinet part of the electrical system, if for some reaon something happens (i.e a 12v feed comes in contact with the cabinet) then this 12v feed should effectively blow its protective device as this would be a dead short?
what sort of problems are you thinking you may be getting? is this some kind of audio setup mounted in a cabinet?
bit more info and might be able to help further.
rich