I'm getting ready to change the amp for the sub/center speaker in my 1991 and I started snooping around so see what I'm getting into and I notice a four in square metal box with some wires plugged into it up under my glove box. Now this wouldn't have raised suspicions except for the fact that I have near it a wire cluster with a plug just hanging there. with no where to go it appears. Is this box some sort of aftermarket item or something oem? If oem, then what's my plug that's hanging there for?