I've removed the Battery, and saw behind it, (next to Fender) a Black plastic box, (seems emty) it has rounded corners.

Don't know what it is good for.
I can't see any wires, but i also was unable to remove it, it is removeable by sliding upwards, (but some Plastics of the Battery Cover is blocking the Box)

is it a vacuum reservoir or something like that?

 

Pictures would help.

 

It's a vacuum canister, it stores vacuum so you can still close a/c vents without truck running.

Wayne

 

is it on the firewall? if it is that is where the vacuum solenoids go for the 4x4 front diff.

 

It's behind the battery, 2WDs have it as well so its not a 4x4 only thing.

Wayne

 

I'm pretty sure what he is talking about is an empty piece of plastic that keeps different size batteries from hitting the fender.