Can anyone help me identify what this box is? It’s says “Full Spec” and is connected to the battery both (+) and (-) And has several wires that is grounded. Any help would be greatly appreciated.

 

welcome to the board.

i'm not certain, but it might be a voltage stabilizer.

maybe a closer photo of it might help.

 

Any signs that a large sound system was once installed in the car?

Sometimes elaborate grounding systems are found in RHD cars.

Do the green wires run off to grounding points?

Looks like one is going to the alternator bracket and another to the MAP sensor mounting bracket.

I suspect another is going to the UIM ground.

 

All are good guesses so far. I would only use green on ground if possible. Trace the leads and see where they all go to. Connect a multimeter and do some probing for 12v or ground, if you know how to. Kinda seems like a battery powered (active)ground enhancer/filter. Never knew they existed, hopefully I'll be wrong.

 

Guarantee that's a voltage stabilizer / grounding kit.
Usually snake oil, but sometimes they help. When I moved my battery to the trunk in my WRX it was absolutely necessary to have my Apexi super grounding kit running. It kept my voltage at 12.7v constant, without it would bounce from like 9-14 and I'd have tons of gremlins. As for "Full Spec", it doesn't show on any Googling, I'd assume all that's inside is probably a resistor and an LED, but it may give you some extra Grounding - which can be overkill if your factory grounds work properly. Like Jeff76 said, if you're curious some multimeter work can reveal if it's in fact doing anything at all.