Left rear? I believe it is the charcoal canister, which absorbs/stores gasoline fumes from the tank and feeds them to the engine while you drive, to reduce evaporative emissions.
My non-ABS Vibe has a rectangular box to the left of the muffler. I'll check for hose connections to the tank in support of my charcoal canister theory.
Thanks MadBill, I think your reply seems to be the most relevant. As I don't have an ABS system, I don't think GM would put an empty black box for nothing.Thanks for everyone's input... I appreciate it!JF