Removing my Toyota Avalon rear speaker.?

Hey every one, Im tryin' to remove my rear speaker I would say its the bass speaker in the very back. I've looked under the trunk and i see some square lookin bolts which i would say they are welded or somethin, so i can't remove them. what should I have to be done in order to remove it? its a 96 toyota avalon.Any one out there who can help me out? I would really apreciate it.
 
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