Alec has a great point. I forgot about the GPS of Onstar....I bet you that is what the box is for.

First, , I'll tell you what those 2 "boxes" in your picture are, they're both On Star. The left one is the "Vehicle Communication Unit (VCU)" and on the right is the "Vehicle Interface Unit (VIU)."

Bellow in my pic you can see the Bose amp on the far left bellow the "Rear Integration Module. Now I'm not 100% sure. But, there may not be an amp on a "non-Bose" system. I have the GM disc for MOD that has my 2001 Bonneville and the only amp it shows is the Bose Amp.

Maybe some one has that information.